Usual reasons for slowdown in an existing computer.
1) Too much junk running in the background. Viruses, too many printer drivers or other junk, crypto mining software, torrent seeding, updates happening (especially if the computer had not been turned on for a while -- Windows update is a system grinder and has been for years). Clear all that out and get the updates done.
2) Overheating. Dust or slipped cable is blocking things, thermal paste/pad dried out to the point of uselessness (rare, and it is usually bad application of a replacement but could be a thing). Check for dust first, try not to spin the fans around when blowing it out with canned air and I would usually avoid a vacuum cleaner as well (static + computers is bad news).
3) Driver update caused issues. This is normally more with new games as the game devs and driver company dance around with each other to sort issues but can happen to older ones. Roll back to older ones, update to newer/beta ones... something to change it and see.
4) Game update changed something. Maybe enabled higher resolution, higher shadows, some fancy physics option or the like. Drop settings back to minimum and then start adding things in from there.
5) Hard drive on the way out. If the drive is constantly remapping sectors or something then this translates as lower speed and as hard drives are the slowest part of the computer...
Can be hard to tell, especially on SSDs, but if something like
https://crystalmark.info/en/software/crystaldiskinfo/ or whatever other S.M.A.R.T. reader you care to employ shows things like remap counts, failed sectors, rewrites as tipping into failure ranges then maybe. Other things like TRIM on SSDs should be handled by the OS these days (earlier OSes might have needed a bit of help getting there), the maker of the SSD might also provide a tool with more insight.
Some other hardware fault (have had RAM modules fail or pop out before thus halving or worse the RAM, and other things overheat) could be causing it but that is super super super rare and likely would have been noticed in other ways. I doubt it would be something like exploit mitigation either (there were some critical bugs found in processors that impacted security, with the fixes needing a bit of a performance hit) as again we are now some years out.