As I see, FOnline is using a lot of calculations, transparency, lights and view fog of war (based on perception I think). So as now is added more advanced lighting and critters transparency - fading in and out as You are moving, this is possible it can run slow. As it is 2D game - with more advanced engine than FO2 and your GPU is advanced in 3d, that maybe problem. It is same like with some CAD, painting programs like Photoshop, etc. I guess FOnline will work best on some Quadro cards

Also check your GPU config and drivers. FOnline is even worth a new pc

Ehh it is too late for me sorry for mistake

I have same issue, so I downloaded FOnline client in zip, and installed it in new directory, then updated. I don't know why, but in previous dir even after removing all files, FOnline won't work, once I get only "error" messages on UI, once some fire dummies and artifacts in gfx, sometimes alert "master.dat not found", and sometimes nothing. After instaling to different dir it work great.