[PD] in which context is GEM slow ?

hello

I would like to use GEM for programming experimental user interfaces. The problem I have is that sometimes GEM works very well, whereas sometimes it's very slow.

For example, when I draw only 4 white mouse-movable squares over a black background, in a 300x300 window, the performances are good (in full screen too). But if I replace the background by a small PNG image (about 12 ko), GEM slows the program a lot (there are cuts in the audio stream). This happens under Windows XP with a recent computer and a usual graphic card. Is it normal ?

In which cases does GEM become slow like this ? Should I rather use GEM under Linux ? Or, maybe GEM is rather adapted to Open GL than image or video matrix processing ?

Thanks for your help