when it gets a bunch of bytes all at once it's not keeping up. MIDI is 31250 bauds, or a byte every 330us or so. When several complete message arrives, my code is not keeping up
I am happily humming along at 115k with my "non-OS OS". I have a "main" and a priority task accessed by setting the bit causing a low priority interrupt. If you have a problem with 31k at 12 MHz you have real trouble in switching lag.

Erik