For what it's worth, we just measured the interrupt latency on our MPC8260
(embedded powerpc) port.  At a CPU clock of 166 MHZ, the highest priority
interrupt service routine begins to run within 5 to 12 usec after the event.

In our port, we reworked the interrupt routing code for generality; you
could probably do better if the routing code was optimised for speed instead.

Regards,

David Querbach
Real-Time Systems Inc.