You should not try to update the display at a higher frequency than you get measurement data.

I don't know the full range of the pulse frequency, but think I would prefer to not count pulses in a window, but instead measure the time between the pulses.

For low speeds, you will get one display update for each pulse received. At high pulse frequencies, you can switch the display output frequency based on two pulse periods, three pulse periods, ... Using multiple pulse periods means you get an averaging and a smoother displayed speed. At the lowest frequencies, you don't have such an option, but will just have to do your best - wait for the next pulse and figure that if it tays longer than a configured limit, you may give up and assume zero speed.