I want the output state to change as fast as possible with a change in the high/low switch state, so I presume an interrupt is appropriate there.
No, interrupts are not appropriate. You're talking about a processor that is capable of executing approximately 16 million instructions per second. Unless there is a bug in your code, from your perspective, switching the high/low state will be instantaneous. Using interrupts always adds a great deal of complexity and, for the task you've described, provides absolutely no benefit.