Timing is *everything* in XIM3 (I've mentioned this before many times). Input from XIM3 is exactly synchronized with the console. The firmware on the device is "bare metal" (no OS or generic USB stack). It's designed such that talking about mouse USB poll rates are irrelevant.
So, to answer your question, the time it takes your button press or mouse move to be "translated" and shipped off to the console is 12ms -- guaranteed.
If fact, if XIM3 *ever* misses an input event or ever misses a report back to the console, it'll acutally display an error and freeze. This way, we'll know if it ever happens in the field. If it does happen, it'll generate support calls and cost to me, but, I rather this happen and we address the problem rather than the customer having a sub-optimal experience.