Jumping fps issue.

Standing in town, doing nothing, nobody was there, almost nothing was moving I constantly have fps jumping from ~150 to ~10. Same problem everywhere else, even more frequent jumps during combat.
http://i.imgur.com/Cmh5HzV.jpg

Enabling VSync decrease frequency of fps drops, but problem still remains.
http://i.imgur.com/rtDk4Nj.jpg

Graphic settings: resolution 1600x900 windowed fullscreen (same problem on other options), every setting on minimum, multithreading enabled, DX11 (same on DX9). Network mode: Lockstep (same on predictive). Sound settings: everything on minimum. Windows 10 64 bit version 1703, compilation 15063.540.

PC: graphic card GTX 660 (driver version 384.94), 8gb of ram, CPU Intel G4500, game on SSD drive. I know it's not the best pc but the game should work very well, doesn't it?