I guess so (haven't that much experience with gem). only know that a small square with some data structures with geographical positions took at least 10% cpu in my thinkpad. with gem it took nothing.
you mean in opengl?
I think this would then be probably done in vanilla OpenGL, not Gem. And I agree this could potentially work but is perhaps not as easily implemented as with using JUCE where a lot of APIs are already prebuilt for you. The same may be the case with Qt but I don't know for sure as I haven't messed with it very recently.
Ico