Computer running slowly
A slow computer is usually a full one, a busy one, or an old one. Only the last needs replacing.
What's usually going on
Computers slow down when the drive is nearly full, when too many programs start themselves in the background, or when updates have been put off for months. Programs that promise to speed up or clean your computer usually make it worse, and some are scams in their own right.
Worth trying first
Restart it properly.
Not sleep, not closing the lid. Choose Restart. If it has been weeks, this alone often fixes it.
Check the free space.
On a Mac: System Settings, General, Storage. On Windows: Settings, System, Storage. If less than about a tenth of the drive is free, that is very likely the cause.
Install the waiting updates.
Updates that have been postponed keep trying to run in the background. Let them finish, then restart once more.
Trim what starts by itself.
On a Mac: System Settings, General, Login Items. On Windows: Settings, Apps, Startup. Switch off anything you don't use every day.
Don't install a cleaner.
Ignore ads and pop-ups offering to speed up, clean, or optimize your computer.
When to hand it over
If it's still slow after all that, if the fan runs constantly, or if it's slow and also showing pop-ups or strange behavior. We'll find out whether it needs a clear-out, a repair, or replacing, and say so plainly.
Your own technologist, on call.
Tell us a little about your setup. We'll take it from there.