Yes, when you're using DT-Ints with an PBP-type ISR it has to save all the system variables when entering the ISR (and restore them on exit). The exact amount of time that takes can probably be different depending on device, if you're using LONGs, what PBP commands are being used in the program and so on. I've measured it a couple of times and the notes I can find at the moment says that the latency is around 20us on an 18F4431 running at 40MHz with a program compiled using LONGs.

20us at 40MHz is 160 instructions.

/Henrik.