QGraphicsView very slow under Linux and Mac OS X

QGraphicsView is apparently very slow under Linux and Mac OS X. I've attached an example program (runs with PySide or PyQt). The signals should be updated every 25ms; the actual timer intervals are displayed on the console. Now under Linux and Mac OS X, the graphics get slower with increasing window size. In full screen, the timer intervals are around 100ms instead of 25ms on my machines.

This problem does not exist under Windows. Here, the intervals are not affected by window size, and the intervals stay at the given values. Interestingly, the program runs perfectly fast even in a Windows inside a VM.

Furthermore, this problem is PySide (or PyQt) specific, because this behavior does not occur when I use Qt from C+. In C+, the program works as expected on all three platforms.