First and foremost, backup anything important. You should have done it yesterday or a week ago, or pretty much any time before now. Its not too late. Back it up.
The fact that you reinstalled should mean the data's somewhere safe already, and that its likely not the OS. Drives do slow down with age, and the apparent lack of other issues makes me disinclined to assume its something else. I'd suggest not using a third party AV - defender/MSSE should be good enough, while you're troubleshooting, or even none at all (keeping the system off network while testing). Have as close to a clean install as possible
At this point, a good idea once you've backed up would be to check your drive's health. While these tools are in no way definitive, it would help you work out if your drives are at fault.
The symptoms indicate its a good time to replace your drive, assuming you can get the data out. We'd also want to check the disks to be sure, but replacing the current boot disk is a good idea anyway.
There's two basic things to do here - checking to see how fast/consistent your drive is, and whether its reporting any errors.
Firstly its useful to rule out any obvious errors with the disk - My preferred tool here is gsmartcontrol
Double click on the attributes tab and look for anything highlighted
Run a short test. If it fails, your hard drive needs to retire. Then run the long test just to be sure (this takes several hours, so worth saving the time if its a dead drive)
I'd also look up the datasheet for your specific drive, and its data transfer speed, then run a tool like the free version of hdtune or crystal disk mark (which I recommend the portable version of). You should be seeing speeds of up to 100-150mb/s peak, with a gentle curve down. Drives do slow down with age, so if you're seeing very wierd,readings, you may need to change your drive too.