What causes computers to slow down?

I'm going to break down this question into mini questions and suggestions to solve them.

• What causes PCs to get slower?

There is this really annoying thing that happens to Windows named “Windows creep”. This is caused by a wide variety of different things. Best way to fix this is to do a full system backup, and reinstall windows.

• Is this a software issue?

Maybe. Check if you have background processes that are hogging system resources, and clear out programs that you don't use. Also check if your PC has a virus, those are often processes that run in the background, and when killed will often spring back to life.

>> Also you can read: Can someone write a computer virus that causes physical harm to the computer user

• Is this a hardware issue?

Though to answer. Do you leave your computer on constantly? If so, you may want to give it some downtime, or a good restart. If your PC runs a HDD then upgrade to an SSD or SSHD if stuck on a budget. Also, if your machine is 64 bit then get at least 8gb of RAM (or whatever your PC's max RAM is)

• Is there a dust clog?

Yeah probably. If you haven't cleaned the dust out of your system then do it. Simplest is to flip your computer over, and find the round cutout with the fan in it, grab a vacuum and some canned air, and get the dust out. If you know what you're doing then fine into the computer and find the fan and clean it.

>> Also you can read: Can a computer virus cause a server to catch fire

• What about Minecraft?

Upgrade the RAM, and get a SSD (if you can) and install a mod called Optifine. Follow tips/tricks videos on YouTube to squeeze out every drop of performance out of your PC. The best fix would be to buy a better PC, but if you're stuck for now this should help.

Edit: Also allocate more RAM to the game (done in the launcher) Most suggest allocating 1/2 of your PC's RAM. I.e 8gb - 4gb allocated, 4gb - 2gb allocated. Go to Google to find out how to do it.

Side note

The PC I use is an Aspire 7745z from mid 2009 with a Pentium P6100. The system has upgraded RAM (8gb), and an Intel 730 SSD (512gb). It is capable of running Minecraft at a respectable (for the computers age, and hardware) framerate of 40–60 FPS average, drops to 30 FPS if I open my browser etc. This is with Optifine, and the settings I chose.