I would agree that you probably can't literally hear the delay among five note-on messages if nothing else is happening. That'd be about .5ms which is pretty insignificant.

But it does become a real thing if you're sending a lot more than that around, like pitch, velocity etc.

It doesn't affect the music industry much because MIDI is overwhelmingly a software standard not a hardware standard these days, so the low baud rate doesn't apply.

Edit: Wait, no, 5ms which is heading towards noticability but not there yet.

Each midi note on and off always sends both pitch and velocity.

The first byte tells you it is a note on or note off and the MIDI channel, the second byte tells you the pitch and the third byte tells you the velocity.

I should have said pitch bend and aftertouch.