Hello there!
I have a problem and can’t find the cause maybe someone is able to helb me.
System: GPU: GTX960 2GB; CPU: intel i7-4790; RAM: 16 GB DDR3
Most of the time, my FPS are between 14 and 25 on low, 1080p.
Sometimes however, my game runs really fluid, even reaching more than 60 FPS. Changing to very high settings results in around 30 FPS, which is a harsh difference to the first scenario.
I already tried uninstalling all mods and Reshade, which doesn’t seem to be the cause.
To deliver some more information:
Around game release: FPS were around 20 with spikes down to 4 in the fight before Runt.
I decided to set the resolution to 768p, which helped, but looked annoying. FPS were about 25, mybe, sometimes went down to 10-15. (Reshade installed) I stopped playing for a while.
Then, shortly after patch 1.3 (I think) I had more than 70 FPS suddenly. I set the resolution back to 1080p and had around 40+ FPS (I was really amazed how well they fixed the FPS problems). When I restarted my PC the next time they were between 15 and 25 again.
Today I started to research and find the cause, uninstalled all mods and Reshade. I made some research and found out that having 4 sticks of RAM at the same time is a bad thing ( 
) , so I put one kit out (2x4GB) and started KC:D.
FPS on 1080p, low: between 55 and 65 FPS; very high: around 30 FPS.
-Restarting the game in the same session made no difference
-Rebooting using the reboot button made no difference
Maybe I should have let it at this point, but smart me really wanted those RAM bars in the first slots (1 and 3). They were at 2 and 4 before. (Completely removed power and deactivated the power supply, of course)
After this reboot my FPS were low again, between 8 and 15 on very high and 20-30 on low.
Installing the RAM in slot 2 and 4 made no difference.
The problem don’t seem to be my system specs, since it can run smoothly, as I witnessed.
I do consider reinstalling the game completely (although the problem seems to have existed from the beginning), but we have problems with our internet connection at the moment.
I stand before you, fellow Henries, having no idea what causes the problem and you asking for ideas on how to fix this matter. 
Greetings, Dejmos