[Ballas]

The few open source implementations I have looked at scan the matrix as fast as the processor allows. So using interrupts can reduce power consumption a lot, but depending on the actual matrix layout, probably not any worthwhile speedup in the nominal case, as you still have to scan the rows after the interrupt.

AFAIR, the USB poll rate is a function of USB rate, and what the endpoint reports/requests. Low-speed, full-speed and high-speed all have different poll rates.