In order of likelihood: a mechanical hard disk, too little memory, too many startup programs, or a nearly full drive. Replacing the disk with an SSD transforms an old machine more than anything else you can buy.
- Check what disk you have. If it is a spinning mechanical drive, replacing it with a solid state drive is the single biggest improvement available, often for well under a hundred pounds.
- Check your memory. 8GB is a working minimum now and 16GB is comfortable. Memory is cheap and often user-replaceable on a desktop.
- Cut the startup list. Task Manager on Windows, Login Items on a Mac. Most of what loads at boot does not need to.
- Free up disk space. A drive above about 90 per cent full slows down measurably.
- Restart it. Machines left on for weeks accumulate problems that a restart clears.
Be very careful with "PC cleaner" and "speed booster" software. A large proportion of it does nothing useful, some of it is adware, and the built-in disk cleanup tools do the legitimate part for free.
A mechanical disk does perhaps a hundred operations a second where a solid state drive does tens of thousands. Everything an operating system does involves waiting for the disk, which is why that one component dominates how fast a machine feels.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.