$2 micros are many MHz. All it has to do is save/compare one timestamp per interrupt.

So lets do some back-of-the-napkin calculations and say it takes 1us to store/compare the timestamp, a switch transitions 50 times per press on each change of state, and a person is typing at 100wpm (lets say 600cpm = 10cps).

I think these are pretty conservative numbers, 50 bounces sounds like a pretty crappy switch to me.

So 1000 transitions per second which would take 1ms, which means 1/1000th of the time is spent servicing the interrupts.

It's not going to cause a problem unless the coding is very inefficient.

Although in many cases polling is fine as well.