For those of you who care:

Using DT-INTs and running an 18F8723@40Mhz -

An ASM interrupt consumes about 12uSec (determining INT source and context saving and context restoring).

A PBP interrupt consumes about 25uSec (more variables to save/restore).

Note that this is just getting into, and out of, the interrupt. Anything you do inside the ISR will add to the times above.