Reinstalled the game today for the expansion and the performance is terrible, playing on all max besides shadows, 20-25 fps in novigrad down to low 30's in the wild, was pretty smooth before didn't even experience these kind of drops with the more heavier reshade profiles. I only just break 40 fps in novigrad putting everything to low and that is with no people in the streets.
If using GOG Galaxy, go to More-manage and click verify/repair or when updating for that matter. Make sure your User.ini is not set to 'read-only' before clicking verify/repair/updating. If there is anything missing/changes made or not ,you will see a pop up letting you know. If you had tweaked settings in your User.ini go check that everything is still the same or not. re-check read-only so that any tweaks you make aren't overwritten. (If you're already aware of this, carry on)
Another thing, GOG ver. as well. Look in your Witcher 3 directory/bin/x64 if you see d3dx11.dll, remove it. If your paranoid, copy and paste it to somewhere safe before deleting. Let me know if any of that helps!
*I'd double check any place you had mods/reshade installed, re-check your User.ini - Also, silly as it may sound, make sure all of your graphic settings are the same/compare to your ini.
Steam also has a function of verify/repair. If none of that works I would try doing a fresh install. Again, make sure your User.ini is not set to read-only while verify/repair. Another thing you might try is deleting your User.ini, when you fire up the game it will set/reset everything to default - As it's supposed to be per the 1.10 patch. Good luck!
Last but not least - IMO, the game looks better, with that and other changes and improvements will come a performance hit. What you speak of sounds like something else however. I almost forgot, double check Nvidia control panel - Have you recently updated any drivers?
ETA - When was the last time you played? They have been tweaking with every patch and this could easily be playing a part in your performance drop. Go in game and use a monitoring program like MSI afterburner and look at your CPU/GPU utilization. Since patch 1.10, Witcher 3 CPU utilization has been higher compared to previous patches (in my experience)
GPU has always been high, at times it makes my 980 sweat!
For reference - Nivida 353.30 drivers. GTX 980, 8gb RAM and i5-4670k and I cannot maintain 60 at all times, use a tweaked Ultra 'Shadowmapresolution at 2048' and I get dips down to low 50's depending on NPC count, weather, location etc.
