Qwik 
“It was fast when I bought it.” True, and it can often be fast again. Here are the causes in the order we come across them, with what you can check yourself.
1. Too many programs at start-up
Every installed program likes to launch itself. On Windows, open Task Manager (Ctrl + Shift + Esc), “Startup” tab, and disable what you don’t need at start-up. On Mac: System Settings, General, Login Items. Nothing is uninstalled, and it can be undone.
2. An almost full disk
Below ten per cent free space, everything slows down. Empty the recycle bin and the Downloads folder, move videos to an external disk, and use the system’s built-in clean-up tool. There is no need to install software for this.
3. A mechanical hard disk
This is the most common cause on a computer more than six years old. If you hear the disk grinding and the light stays on constantly, that’s the culprit. Replacing it with an SSD changes everything: start-up in twenty seconds instead of three minutes. It is not something to do yourself without a backup, but it is the most cost-effective repair there is.
4. Not enough memory
With 4 GB, a browser and a video call are enough to saturate the machine. 8 GB is today’s comfortable minimum. On many PCs, memory is easy to add; on thin laptops and recent Macs, it is soldered.
5. The browser
Thirty tabs open and ten extensions, three of which you never installed on purpose. Close them, uninstall unknown extensions, and see whether the slowness goes away.
6. Pending updates
A Windows machine that is downloading and preparing updates in the background crawls for hours. Let it finish once and for all, restart, and repeat until there is nothing left to install.
7. A virus, sometimes
Less often than people think. The signs: adverts that open on their own, a changed home page, a fan running flat out for no reason. Run a full scan with the built-in antivirus.
What is best avoided
- Free “cleaners” and “speed boosters”: useless at best, often the problem themselves
- “Registry cleaners”: no gain, a real risk
- Calling the number in a window announcing an infection: it is a scam
- Reinstalling Windows without a backup
When to call
If the disk grinds, if the computer heats up and shuts down, if it freezes even when idle, or if none of the above has changed anything. A diagnosis tells you within an hour whether the machine deserves an SSD, a clean-up or a replacement.
In short: start-up, disk space, updates and the browser can be sorted out on your own. A mechanical disk and memory require a screwdriver. See Viruses & slow computer.