how to optimize the interrupt latency...
you cant 'optimize', when taking 'latency' litterally. Interrupt latency is a hardware function

what you can do is:
.. make the critical one a higher priority (still nothing to do with 'latency'
.. avoid the few 3 and 4 cycle instructions (derivative dependedant), impossible in C
.. KISS (Keep Isrs Short and Simple)
.. use faster derivarive (e.g. a f12x/f13x at 100MHz)

Erik