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Slow computers, frozen apps, full storage and crashes

Work out why a computer feels slow, close a frozen app safely, find out what is filling your storage and respond calmly when a program crashes, on both Ubuntu and Windows.

30 minelementaryUbuntu/LinuxWindowsPractical task needs a laptop or desktop

Marking a lesson as read does not prove you can do it. Skills count as demonstrated when you pass the quiz (not yet) and complete the practical tasks (not yet).

By the end of this lesson you will be able to

  • Name the usual causes of a slow computer and test them one at a time.
  • Close a frozen application with Task Manager (Windows) or System Monitor (Ubuntu) without losing other work.
  • Check how much storage is free and find the biggest space users without deleting anything risky.
  • Respond to a crashing program by gathering evidence and trying safe fixes in a sensible order.
  • Decide when a frozen or slow computer needs a restart or outside help.

A slow, frozen or crashing computer is the most common complaint in the world, and it feels urgent because you cannot do your work. The good news is that a handful of causes explain most cases, and you can test them safely in a few minutes. This lesson uses the troubleshooting method from the last module: look at the evidence, make one change, check the result. Nothing here asks you to delete system files or install 'cleaner' programs.

Why computers become slow

A computer is like a kitchen. The CPU is the cook, RAM is the worktop and Storage is the cupboard. If too many dishes are being prepared at once, the cook is busy. If the worktop is full, things get piled in the cupboard and fetched back again, which is much slower. If the cupboard is nearly full, even finding a place to put things takes time. Almost every slowness has one of these pictures behind it.

Diagram

The four commonest causes of slowness and the first safe test for each.

Too many programs
  • Many windows open
  • Dozens of browser tabs
  • Apps running in the background

Test: close what you do not need.

Memory full
  • RAM is the short-term workspace
  • When it runs out, storage is used as overflow
  • Overflow is much slower

Test: look at memory use.

Storage nearly full
  • Under about 10% free
  • Saving and updates slow down
  • Updates may fail

Test: check free space.

Heat
  • Hot, blocked vents
  • Fan roars
  • Processor slows itself down

Test: clear the vents, let it cool.

The four commonest causes of slowness and the first safe test for each.
Read this figure as text

Four columns comparing too many programs, full memory, nearly full storage and heat, each with a first safe test.

Four columns. Column 1, Too many programs: many windows, browser tabs and background apps compete for the processor; first test is to close what you are not using. Column 2, Memory full: the computer runs out of RAM and starts using storage as slow overflow space; first test is to look at memory use in Task Manager or System Monitor. Column 3, Storage almost full: less than about 10 percent free makes saving, updating and starting slow; first test is to check free space. Column 4, Heat: a hot laptop slows its processor on purpose; first test is to feel for hot air, listen for a loud fan and make sure the vents are clear.

A calm order for fixing a slow computer

Start with what costs nothing and cannot do harm. Work down this list and check the speed after each step. Stop as soon as the computer feels normal again.

  1. Save your work and close programs you are not using. Close browser tabs too. Each tab is a small program of its own.
  2. Restart the computer. A restart clears out memory that has filled with leftovers. It also finishes updates that were waiting. This single step solves a surprising share of cases.
  3. Look at what is using the resources with Task Manager or System Monitor (shown below). One program using nearly all the processor is a clear clue.
  4. Check free storage (see the storage section).
  5. Check heat. Is the laptop on a bed or sofa, blocking the vents? Move it to a hard, flat surface.
  6. Check for updates. Some slowness is a known fault that an update fixes. Make sure the battery is charged or the charger is connected first.
Diagram

A decision tree for a computer that has become slow.

  • Have you restarted the computer today?
    • YesDoes the monitor show one program using most of the CPU or memory?
      • YesDo thisClose that program. If it will not close, end it from the monitor.
      • NoIs free storage below about 10%?
        • YesDo thisEmpty the bin, move backed-up big files and remove unused apps.
        • NoIs the laptop hot or the fan very loud?
          • YesDo thisUse a hard, flat surface, clear the vents and let it cool.
          • NoDo thisThe cause may be an old drive or too little memory for your work. See the repair lesson in the next module.
    • NoDo thisSave your work and restart. Check the speed again.
A decision tree for a computer that has become slow.
Read this figure as text

A yes or no decision tree that leads from a slow computer through restart, memory, storage and heat checks to a fix.

Start: the computer feels slow. Question 1: have you restarted it today? If no, save your work and restart, then check again. If yes, question 2: does Task Manager or System Monitor show one program using most of the processor or memory? If yes, close that program, and if it will not close, end it from the monitor. If no, question 3: is free storage below about 10 percent? If yes, free space safely by emptying the bin, moving big files you have backed up and uninstalling apps you no longer use. If no, question 4: is the laptop hot or the fan loud? If yes, move it to a hard surface, clear the vents and let it cool. If no, ask for help, because the cause may be an ageing drive or too little memory for your work.

A frozen application

A frozen application is one that has stopped responding to your clicks. The window may go pale, the pointer may show a spinning circle, or the title may say '(Not Responding)'. The rest of the computer usually still works. Often the program is just busy, for example opening a very large file, so the first step is to wait for a minute without clicking repeatedly. Extra clicks pile up and make things worse.

If nothing happens after a minute or two, you can ask the system to end that one program. This is safe for the rest of the computer, but anything unsaved in that program is lost. Be sure you have chosen the right program.

Close a frozen program

Both systems give you a list of running programs and let you stop one of them. The names differ.

Ubuntu: System Monitor

  1. Step 1: Wait first

    Give the program a minute. Ubuntu may itself show a box saying the program is not responding, with the choices Force Quit and Wait.

    You should see: If the program wakes up, choose Wait or simply carry on.

  2. Step 2: Open System Monitor

    Press the Super key (the Windows key), type System Monitor and press Enter. If it is not installed it can be found in App Center.

    You should see: A window opens with tabs named Processes, Resources and File Systems.

  3. Step 3: Find the program

    On the Processes tab, click the Name heading, or the CPU column, so the busiest items rise to the top. Look for the program's name.

    You should see: A frozen program often shows very high CPU use or is listed as not responding.

  4. Step 4: End it

    Right-click the program and choose End Process. This asks it politely to close. If it still does not close after a minute, right-click it again and choose Kill Process.

    You should see: The window disappears. A confirmation box may ask you to confirm.

Simulated interface: not a real screenshot

A simulated Task Manager with one frozen program ready to be ended.

Task Manager
Processes
Performance
Startup apps
BrowserCPU 18%
NotesCPU 1%
Photo Editor (Not responding)CPU 52%
Music PlayerCPU 2%
End task
  1. Marker 1: Processes page
  2. Marker 2: The frozen program
  3. Marker 3: End task
A simulated Task Manager with one frozen program ready to be ended.
Read this figure as text

A simulated Windows Task Manager window listing apps, with one marked Not responding and numbered markers on the list and sidebar.

A simulated Task Manager window with a sidebar listing Processes, Performance and Startup apps. The Processes page lists four programs: Browser using 18 percent CPU, Notes using 1 percent, Photo Editor marked Not responding using 52 percent, and Music Player using 2 percent. Marker 1 points at the sidebar item Processes. Marker 2 points at the row Photo Editor, which is selected. Marker 3 points at the End task button at the bottom. Right-click the frozen program, then choose End task.

If the whole screen is frozen and the pointer does not move, wait a few minutes, because a heavy task or a disk problem may be finishing. Try Caps Lock: if its light does not respond, the system has stopped. As a last resort, hold the power button for about five to ten seconds until the computer switches off, then wait a few seconds and switch it on again. Unsaved work will be lost, and this can occasionally disturb files that were being written, which is why a backup matters.

Full storage

When storage is nearly full the computer cannot save files, install updates or create the temporary files that programs need. You may see messages such as 'No space left on device' or 'Your device is running low on storage'. Aim to keep at least 10 to 20 percent free. The aim is to find out what is taking the space and then move or remove only things you understand.

Check free space and what is using it

Ubuntu

  1. Step 1: See the total

    Open Settings, choose System (or About on some releases) and look for 'Disk Capacity' or the storage figure.

    You should see: A number such as 256 GB of total storage.

  2. Step 2: Find the big items

    Search for Disk Usage Analyzer in the Activities overview and open it. If it is missing, install it from App Center. Choose to scan your Home folder.

    You should see: A list of folders sorted by size, such as Videos, Downloads and Pictures.

  3. Step 3: Empty the Trash

    Files you delete go to the Trash first and still take up space. In Files, open Trash in the sidebar, check nothing important is inside, then choose Empty.

    You should see: Free space increases by the size of what was in the Trash.

You can also ask Ubuntu for a quick summary in the Terminal. The command df -h / means: show the disk free space (df), in human-readable units (-h), for the main drive (/). It only reads information and changes nothing.

Terminal (Linux) · you can run this on your own computer
df -h /
Simulated interface: not a real screenshot

A simulated terminal showing the free-space command and its output.

Terminal (simulated)
  1. Marker 1: Size is the whole drive; Avail is the free space
  2. Marker 2: 24G free and 90% used: nearly full, time to make room
A simulated terminal showing the free-space command and its output.
Read this figure as text

A simulated Ubuntu terminal running df -h with a table of size, used, available and percentage used.

A simulated terminal. The prompt line shows the command df -h /. The output has a heading row: Filesystem, Size, Used, Avail, Use percent and Mounted on. The data row reads /dev/nvme0n1p2, 234G, 198G, 24G, 90 percent, mounted on /. Marker 1 points at the heading row, where Size is the whole drive and Avail is the free space. Marker 2 points at the data row: 24 gigabytes free and 90 percent used, which means the drive is nearly full and needs attention. The numbers are fictional.

Crashes

A Crash happens when a program hits a problem it cannot handle and closes suddenly, sometimes with a message such as 'has stopped working' (Windows) or 'closed unexpectedly' (Ubuntu). A crash differs from a freeze: the program disappears instead of hanging. Your other programs are usually fine.

Diagram

A sensible order of responses when one program keeps crashing.

  1. Step 1: Note itProgram, action, message
  2. Step 2: ReopenIs your work recovered?
  3. Step 3: RestartWhole computer
  4. Step 4: Space and updatesFree storage, install updates
  5. Step 5: ReinstallOne program only
  6. Step 6: AskBring your notes
A sensible order of responses when one program keeps crashing.
Read this figure as text

A flow of six steps from noting the error message to asking for help when a program keeps crashing.

Six steps in order. Step 1, Note it: write down the program, what you were doing and the exact message. Step 2, Reopen: open the program again and see if it recovers your work. Step 3, Restart the computer: clears temporary faults. Step 4, Check space and updates: free storage and install updates for the program and the system. Step 5, Reinstall: if only one program fails, remove it properly and install it again from the official source. Step 6, Ask: if it still crashes or other programs crash too, ask for help with your notes.

Pay attention to the pattern. If one program crashes while you open one particular file, the file may be damaged: try a backup copy. If one program crashes every time, it may be out of date or damaged. If many different programs crash, suspect memory, heat, storage or a failing drive, and see the repair lesson later in this path. Many programs also offer to recover your work when you reopen them, so say yes before doing anything else.

Common mistakes

  • ✖ Instead of: Clicking repeatedly on a frozen window.

    ✔ Do this: Wait a minute. If it does not wake up, end that program from Task Manager or System Monitor.

  • ✖ Instead of: Deleting folders you do not recognise to free space.

    ✔ Do this: Use the Storage settings or a disk-usage tool to see what is big, and move only your own files after backing up.

  • ✖ Instead of: Installing a 'PC booster' because an advert said the computer is slow.

    ✔ Do this: Use the built-in tools. Most 'boosters' are useless or malware.

  • ✖ Instead of: Ending every process that uses a lot of CPU.

    ✔ Do this: Only end programs you recognise and can see are stuck. Heavy use is normal while a program is working.

If something goes wrong

System Monitor or Task Manager is itself slow to open.
The computer is very short of memory or busy. Wait a minute. If nothing happens, hold the power button to restart as a last resort and check storage and memory afterwards.
I ended a program but it comes back immediately.
Something may be restarting it. Restart the computer. If it keeps returning and misbehaving, check for updates for that program and reinstall it properly.
Storage is still nearly full after emptying the bin.
Look for large files in Videos, Downloads and Pictures, and old programs you no longer use. Back up, then move large files to an external drive. If you cannot find the cause, ask for help rather than deleting guesses.
A program crashes only when I open one file.
The file may be damaged. Try an earlier copy from your backup, or open it in another program.

Key terms

Frozen application
A program that has stopped answering your clicks while the rest of the computer may still work.
Crash
When a program meets a problem it cannot handle and closes suddenly.
Task Manager
The Windows tool that lists running programs, shows how busy the computer is and lets you end a stuck program.
System Monitor
The Ubuntu tool that lists running programs, draws graphs of processor, memory and network use and lets you end a program.
Temporary file
A file a program makes for short-term use that is usually safe to remove once the program is finished.

Practise it for real

Reading is not the same as being able to do it. Complete these on your own computer, then tick only what you have actually done. These ticks are your own honest record: nothing checks them for you.

Guided exerciseNeeds a laptop or desktop

Find the busiest program

Open Task Manager (Windows) or System Monitor (Ubuntu). Open three programs, for example a browser, a text editor and the file manager. Sort the list by CPU, then by Memory. Write down which program uses the most of each and what the numbers are. Do not end anything. Then close the programs again and watch the numbers fall.

You have succeeded when:
Independent challengeNeeds a laptop or desktop

Challenge: make a storage survey

Without deleting anything, find your drive's total size, free space and percentage free, and list the three biggest categories or folders. Write them in a short note. Decide whether your free space is healthy (more than 10 to 20 percent), and write one safe action you could take if it were not, for example moving backed-up videos to an external drive.

You have succeeded when:

Knowledge check

6 questions. You need 80% to pass. Mistakes here are part of learning, and you can retry as often as you like.

  1. Question 1.A laptop is slow and Task Manager shows memory almost completely used. What is the likeliest effect?
  2. Question 2.Put these fixes for a slow computer in the order you should try them.
    1. Save work and close unused programs
    2. Look at what is using resources
    3. Restart the computer
    4. Check free storage
    5. Check for heat and clear the vents
  3. Question 3.When you end a frozen program from Task Manager or System Monitor, unsaved work in that program is lost.
  4. Question 4.Wanjiru's laptop says 'Your device is running low on storage'. She sees a large folder called Windows in File Explorer and wonders if she can delete it to make room.What should she do?
  5. Question 5.Match each description to the right word.
  6. Question 6.The whole screen is frozen and nothing responds after five minutes. What is the correct last resort?

Summary

  • Slowness usually comes from too many programs, full memory, nearly full storage or heat.
  • Try free, safe steps first: close programs, restart, then look at the monitor.
  • Wait before ending a frozen program, and end only a program you can see is stuck.
  • Keep 10 to 20 percent of storage free, and never delete system folders to make space.
  • For crashes, note the message, restart, update, and ask for help if many programs crash.

Checklist: can you do these?

Marking a lesson as read does not prove you can do it. Skills count as demonstrated when you pass the quiz (not yet) and complete the practical tasks (not yet).

Related lessons

References and further reading

Official sources for details that can change between versions.

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