Summary
This guide helps you diagnose and fix a computer that feels slow or heavy on Windows or macOS. Follow the steps in order. Each step narrows down the cause and often fixes the issue without needing IT.

Symptoms
Apps take a long time to open
Typing lags behind what you press
Clicking feels delayed and you may see a spinning wheel or not responding message
Web browsing is sluggish
Fans are loud or laptop feels hot
The computer is slow right after logging in

Likely causes
Too many background apps running
Low free storage
Updates finishing and indexing or search rebuilding
Cloud sync uploading or downloading lots of data
Too many browser tabs or extensions
Thermal throttling from overheating
Malware or unwanted software
Failing storage drive or disk problems

Fix in order

1 Restart properly
A proper restart clears stuck processes and completes updates.
Windows Start menu then Power then Restart. Do not use Shut Down.
Mac Apple menu then Restart.
If the system feels normal after restart, you are done.

2 Check free storage
Aim for at least 15 to 20 percent free space.
Windows Settings then System then Storage then Temporary files. Remove safe items such as temporary files, recycle bin, thumbnails, and delivery optimisation files.
Extra quick wins uninstall unused apps in Settings then Apps then Installed apps. Move large files such as Videos and Downloads off the main drive if possible.
Mac System Settings then General then Storage. Use Recommendations and empty Trash. Remove large unneeded items, especially from Downloads.

3 Find what is hogging CPU memory or disk
Windows press Ctrl plus Shift plus Esc to open Task Manager. Go to Processes and sort by CPU, Memory, and Disk. Look for one app using very high CPU for minutes, memory near 90 to 100 percent, or disk stuck at 100 percent. End task only if you recognise the process.
Mac open Activity Monitor from Utilities. Check CPU, Memory, and Disk. Look for apps stuck at high CPU, memory pressure showing yellow or red, or constant disk reads and writes. Quit or force quit only if needed.
If one process is clearly the problem, you have likely found the root cause.

4 Disable unnecessary startup and background items
Windows Task Manager then Startup apps. Disable anything non essential such as game launchers, toolbars, or helper apps. Leave security tools and required company tools enabled. Restart after changes.
Mac System Settings then General then Login Items. Remove anything you do not need starting automatically. Restart after changes.
If the computer is much faster after restart, startup bloat was a key factor.

5 Browser cleanup
Browsers are a common performance cause.
Close unused tabs and keep the number of tabs reasonable.
Update your browser.
Remove or disable extensions you do not need, especially unknown extensions or coupon finders.
If performance is still bad, try a private or incognito window. If that is faster, extensions are likely the issue.

6 Cloud sync check
Cloud sync can saturate disk and network.
Pause OneDrive, iCloud Drive, or Dropbox for 10 to 15 minutes.
Use the computer normally. If it becomes fast, syncing load was the cause.
Leave large uploads for off hours and avoid syncing giant folders unnecessarily.

7 Updates and indexing
After updates, indexing and background tasks can cause temporary slowness.
Windows Settings then Windows Update. Install pending updates and restart. Let indexing finish.
Mac System Settings then General then Software Update. After major updates, indexing may run for a while and performance improves when it finishes.
Rule of thumb if the machine is slow right after an update, leave it plugged in for 30 to 60 minutes, then restart.

8 Malware and unwanted software scan
Windows open Windows Security then Virus and threat protection then run a Full scan. If available, run Microsoft Defender Offline scan.
Mac remove suspicious browser extensions, check Applications for unknown tools, and run your approved endpoint security scan if deployed.

Prevention
Restart weekly.
Keep at least 20 percent of the disk free.
Limit startup items.
Limit browser extensions and tabs.
Avoid downloading unknown optimisers or toolbars.
Let updates complete and do not force power off during updates.

When to contact IT
Disk errors, clicking drive noises, corruption, or repeated repair prompts
Repeated freezing even after following the steps
CPU is high at idle when nothing is running
Performance suddenly drops overnight or becomes severely worse
Laptop overheats or shuts down