This problem with fps only happened to me when i use Schwarzer Reiter skills (FPS drops to 1~5), only this class have the fps problem (to me) because i can play with all others normally.
It also happens to me when I am currently equipping an Arde Dagger while using SR skills. The game is switching the dagger to the pistol when you use a skill due to the autoswap, which seems to be causing the lag. If I just equip a pistol, there is no lag at all. Are you using a dagger while using SR skills? If so, you need to swap manually to a pistol to prevent lag when using the skills.
I really thank you, it’s exactly what happened to me 