I run a fairly decent rig and get 80-90+ fps with medium-high settings, when I play LSPDFR I get 40-50, no sweat... last time I played everything was fine, it shut down normally and everything, and then I go to use it yesterday and the base game is at 19-25 fps?! HUH? Ok, maybe I screwed up, so I start pulling mods out, then I even made a fresh install overnight, still nothing. The highest I've seen was 40, but that was a brief second, I'm running low 20s now base game no mods, fresh install.... driver's are all updated, other games are still at 60+fps... any tips before I fresh install windows?

All I can say is R.I.P😄 Sounds like a tough problem. How many times have you restarted your rig? I know it sounds foolish but that helps me with those problems occasionally. I assume you probably have, but you never know. Sometimes also taking out components of your computer and putting them back in helps. I did that with my graphics card and it gave me a few frames back.

In case anybody stumbles across this again with a similar issue, my cooling fan had come off my CPU, so it was overheating and throttling itself to not destroy itself. It's rarely a physical problem, but this time it is. Re-attached the fan and never had another issue.