The most likely problem from my experience would be a bad hard drive. When it gets problems attempting to read bad sectors, then it slows down significantly.
Other problems you could check into include:
1. Not enough power? maybe a degraded PSU no longer gives enough power during high power consumption gaming?
2. Other applications taking up too much cpu/gpu usage?
3. Viruses or remote Bitcoin mining?
4. Maybe just too much stuff.. My 1TB main C drive is full to about 70GB free room, and it's getting seriously snail-ish.