There is so many reason to slow down PCs
1) Virus infection or some kid of activity of malware or virus
2) Operating system relate issue : such as PC running with lots of temporary files and large registry
3) Hardware malfunction or source hard drive full with data : could be SSD related fault or RAMs / VRAM, motherboard data bus related ets..
You can speed up the PC using below steps
1) Clean the viruses malware if available with your PC and remember to keep only one antivirus for the system and make sure it running with latest update.
2) Clean the temporary files using disc cleanup wizard and remove unwanted programs if available or you can install fresh os
3) run the disk check function and check whether bad sectors or hard drive issue observed in the system. clean the hardware and slots and try you could get the speed back
Cheers!