Did you run Windows update or update drivers when the issue appeared? I don't know but because some security issues, CPU performance could go down after updates the last few weeks.

To see is it depends on your system or Quake config, you can run system restore in Windows, pick a date before the issue started.

The power plan can have a big impact, you can change it manually or try the default high performance.

What happens is that for the first time I run the game after a fresh install, it's fine for about a day, then when trying to run the game again the frame drops happen.

Have tried a system restore and it didn't really help, also tried changing power plans and stuff

If you had laptop, I'd say that QL tries to run on integrated GFX card (Intel HD Graphics) instead of external one (nVidia GTX). But if you have PC it is not your case for sure.

update your graphics driver from nvidia website (with advanced install -> clean install option)

Check your CPU's and GPU's power saving settings. Try either setting them to performance or disabling the power saving options altogether.

you could have a bitcoin miner. it happened to me. they're a type of virus that's really hard to track down, and mostly wrecks your GPU. it could have damaged it. check your next electric bill and see if it's suspiciously high. that was the first sign, then i checked through system logs and found the machine was turning itself on in the middle of the night. you can also leave a temp program open and see if your PC is getting really high when you should be idle.

see if your PC is getting really high when you should be idle.

unless you're fizzle, then your pc should be high all the time

just to add to what i said. if you're reinstalling QL and it's fine for a few days, that makes no sense. but if you failed to mention that you are also reinstalling some other game at the same time, in other words, pirated software, it is most likely a bitcoin miner.

when this happened to me, i didn't catch it fast enough and it caused my video card to crap out a few months after i got rid of the virus from the strain. i don't pirate games anymore because of this, it's cheaper to just buy the game than run the risk of blowing your hardware out.

Could be some antivirus scanner that starts running when you start QL? Check CPU usage when QL is running, if it's very high it could impact FPS. It can happen that certain apps can affect certain other apps, including games, uninstall all programs you don't use anymore, could be an idea to stop services you don't use (check first if they are needed).

buy xbox

I had a problem as well for me the fix was to set manual resolution in quake settings instead of desktop resolution (automatic), it was running almost constantly at around 40-50fps and dropping,
This was due to the 4k resolution of my display :/

Now after manual settings its stays at 190-200fps... finally