[notaddicted]

I've got no direct knowledge of this in particular but I've heard that the mouse movement is one of the highest level interrupts in the OS and won't be preempted -- so the number is probably very small but the response by the application when you click may be longer. Also, this is why sometimes you'll see the entire computer locked up except for the mouse movement.

[Amadou]

Part of that is due to having a hardware mouse cursor. Basically all the interrupt handler has to do is load the new coordinates into some registers on the video co-processor and co-processor takes care of all of the work of blitting the mouse around the screen. It makes processing the mouse interrupts very light-weight.

[snom380]

Which OS and hardware platform are you referring to now?

[makomk]

I think this applies to most desktop OSes and nearly all the hardware platforms they run on, aside from virtualised ones.