Serial interrupt RX-C7 crashes after sending TX-C6 char PC

[Ttelmah]

Generally only ever use FAST_IO, if you must. Certain things like byte
wide read of an optical encoder, where other pins are used for output.

The key lesson, if if you use FAST_IO, you _must_ get the settings right.
This means reading the data sheet, for every peripheral, and checking
that the settings are correct to allow it to work.
Now a lot of 'newer' chips, if you enable a peripheral, doing this overrides
the TRIS setting for this. On older chips like this one, this does not happen.
There is a small ROM size increase, but this should be very small. If it is
large, it suggests you have a lot of places where the TRIS is being changed.

For the modern compilers, I'd say that 99% of code should simply sit
with the default standard_io.

[temtronic]

Something else to consider is that modern PICs can run faster, so the time to execute the 3-4 extra lines of code is insignificant in most cases.
I see you're running the 876 at 10MHz, if you go to the max (20 MHz), you'll double the speed (simple xtal change) and be able to get more done in a shorter time.