MIDI standards:
One start bit, eight data bits, and one stop bit result in a maximum transmission rate of 3125 bytes per second
The status byte determines the length of most messages, which are usually one, two, or three bytes in length
so at 3 bytes per message you'd get about 1042 messages per second, which puts each message at less than 1msec per message worst case, with about 320usec best case (that's 320micro seconds, or 0.32millisecs).
Using MIDI thru will make the times add up, but you can overcome that using a MIDI clock driver.