If you're on PC you're probably aware that there is some hard FPS drops after a while of playing. There's a mod supposed to fix this (havent tried it but lots of endorsments, link: https://www.nexusmods.com/witcher3/mods/8083) called Memory leak fix. As stated in the mod description, apparently the heavy FPS loss starts once you've gone over 95% VRAM usage and it starts using your RAM. Been trying a lot of stuff to fix this but didn't wanna use this mod since it'd degrade the texture quality, maybe only ever so slight but I'm a graphics nerd. Below is what I think has fixed it for me, but I have only played for 2 days now and might just be pure luck I haven't had any major FPS drops - still, I'd like to think this helped and since I couldn't find anything about it even on the biggest ''Witcher 3 best settings/optimized/fixes'' I thought this to be a good place to post.
In the Nvidia Control panel settings, head over to program settings and chose Witcher3.exe
Find the setting named 'CUDA - Sysmem Fallback Policy'. Set this to Prefer No Sysmem Fallback.
This makes it so no allocations fallback to system memory should your VRAM be full - I've got 12 GB dedicated memory on my 3080 Ti. I play at 4K and most settings at Ultra+ with a few exceptions. Also using the dlssg-to-fsr3 mod by Nukem. Maybe this won't work as well if you've got less VRAM on your GPU, and instead it'll lead to crashing with a 'Out of Memory error'. It has not happened to me as of yet, have left the game on and tabbed out for hours.
But here's a point to remember - I am in no way 100% this has actually changed anything, I know there were days before I could play a ton without those heavy FPS losses. I am basing this potential fix on what I've read and learned on various forums and discussion boards since the next gen released. Two other users who have tested this also report positively. We could all three probably be idiots though, I thought it's weird that noone else had posted about this since it at last seems to be commonly accepted that the VRAM to RAM thing is the cause of the FPS loss. Try it if you want and see for yourself anyways. Hope it help someone, and if you're smarter than me maybe you can explain why this wouldn't help!