Performance drop on fullscreen?

for a while I have experienced significant performance drop when I switch to fullscreen mode (by the console command “fullscreen” or “FullScreenMode” or “SetRes”), like 50 FPS to 35 FPS, and switching back to windowed mode results in keeping the decreased FPS. I don’t remember exactly in which version I experienced it first, but it can be a driver related issue, it is approx. since upgraded to Win10 half a year ago…
is there anyone else having the same problem?
unfortunately my laptop is quite old and no more driver update out there supported… (AMD 7670M)
thanks for any idea.

Are you also changing screen resolution?

Any game that is GPU bound will experience FPS drop when rendering higher resolution, simply because there are more pixels to process.

no, I keep 1366x768