I’ve had my Nano based G.E.A.R. sequencer running at up to a few hundred Hz clock speed — and it wasn’t maxing out then, although that was before I added some software switch debouncing that would limit that. I wasn’t working the controls in that test, so it didn’t have to process input and display it on the OLED, but it was reading five analog pins on each loop. Interrupts are used to handle clock events.
Earlier I’d found the user interface bogging down the sequencer speed, but that was before I made some software improvements. In particular I ditched digitalRead and digitalWrite in favor of the DirectIO library and that seemed to make an improvement.