Hm... I suspected that
The strange thing is that they run smoothly within small and medium size windows, but it seems as if there is a hard limit (when I resize the window) at which the image suddenly stops and ends up at 0.8 fps (i measured... *sigh*)
In fullscreen, of course, the window is way beyond this "limit". I have not been able to determine exactly how big the window is when this happens.
Somehow this seems to me to be some memory limit or something, but I don't know where to look for more info...