You are right. I did not notice the usleep() you use to synchronize between both event loops. You indeed do not need a QMap to synchronize (a dirty flag will do). The pointer passed along to repaintPixmap() is often the same anyhow.

Also, I wonder if you could speed things up by waiting a bit longer than usleep(1), i.e. 1 microsecond. I don't know how fine-grained the N900's default timer resolution is, but if it would be that accurate, you are spending a lot of CPU with busy waiting. I think somewhere between 1 to 10 milliseconds (or 1000 to 10000 microseconds) might improves things as you sleep longer per iteration, leaving more time for the other threads to complete the repainting. I have not tested this though, as you would have to count how often you execute the cycle in a benchmark setup (where you try to paint as fast as possible).

Cheers,
Davy