If you are still going on it, you probably know not to bother looking at BASIC interrupt in this case.
Usually there is time taken saving and restoring context, but that could likely be avoided
if the program is under 2K, and doesn't care where it was up to when the interrupt was called.

I will be writing for a 16F877 soon, and would see how fast I could just cycle a pin,
but don't have anything to measure it with.