Ah those slowing computers. My biggest headache supporting people.
If you are using windows, you are probably experiencing the inevitable problem inherent in the OS, time and efficient use of drive resource and fragmenting. The OS has had issues, really since day one, one problem is swapping memory to disc and back to memory again. Another issue is installing more software and updates: these will install services and programs that will fill up your memory and force the first issue to occur. The easiest way I have found to fix this is to re-image (re-load) the OS, means wiping out the current drive; I usually buy a new hard drive and keep the original as the backup data drive either internal or external. Only load the stuff you really need, games are usually the biggest system resource user.
If you are using a SSD as your main drive, it could be the typical degradation inherent to these drives. This really depends on the amount of writes have occurred to the drive. The newer SSD drives have improved tremendously and will go years before degradation occurs.
Before you do the re-image note: heat is a CPU's worst enemy. I will always take the computer box outside remove the side panel and with a can of compressed air blow out the cooling fins, the fan blades and blow air through the power supply vents. DO NOT USE A VACUUM CLEANER TO SUCK THE DUST OUT. A vacuum will generate static electricity and in a most efficient manor, completely destroy the video, RAM, CPU and other components. I speak from experience, people have called me and said "I used the vacuum to clean out my computer and now it won't work." I usually recommend buying a new computer, the cost to repair would be more. As a professional I have a small compressor that allows me to blow a strong stream from at least 1 foot away from the components. At the same time I blow out the crumbs from inside the keyboard.
As a bonus piece of info, don't put your computer on the floor, treat it well and put it on a table and dust it as you would other surfaces. The floor is the biggest collector of dust bunnies. If you look around your floor based computer you will see dust bunnies do not exist in front of the computer, they are easily sucked into the computer and nest quite nicely in those computer fans.
Note, laptops are also prone to heat issues, air intake can fill the fins with dust and blocking either the intake or exhaust will cause overheating.
Anyway, I hope this information is useful.