Is there any additional latency, compared to other interrupts, between an external interrupt pin triggering and the associated interrupt handler being called?
Yes. External interrupts incur an additional four clock cycles of latency. This is due to de-glitching and synchronization flops on the external interrupt pin. This means that, generally speaking, a normal interrupt will have a minimum of 12 clock cycles before reaching the interrupt handler, while external interrupts will take a minimum of 16 clock cycles.