Well that's a loaded question. First thing I'd suggest is to download Malwarebytes. There may be a chance you've been infected with something and it's causing the slowdown.
If that's not the case, have you tried doing a clean install of Windows? Generally, if you just upgrade Windows, it leaves behind a lot of junk that will slow down your computer. The easy way to do this right now is back up everything on your PC and go to Start --> Search for "Reset this PC". Then follow the process to do this. This will completely reset your system and it should speed things up by a lot. If it doesn't help, you may want to look into upgrading your PC. Something like a SSD if you haven't installed one already will give you a good boost of performance.