I assume the only problem with increasing the crystal frequency is that some of the PICBasic
default commands assume 4Mhz, so care would be needed when implementing them?
you'll void some problem when you'll add DEFINE OSC (value) at the begining of your code. But if you plan to use any kind of PULSIN or else... sure you have to be carefull.


However, there seems to be a suggestion on various sites that an ASM service routine embedded in a Basic program will be called more quickly - why should this be?
for the same reasons as above. But don't be afraid for now. Try it first. BASIC interrupt is not as slow as you can think. Your coding style can slow the interrupt.

BTW, feel free to post any kind of code here if you have problem.

Happy new year to you!