FWIW, some data using the “thump test” from my system, 5 thumps, with the following configuration (44.1K, 256 sample buffer):
a) Yamaha Motif synth audio directly to interface audio in (M-Audio Delta 66 card, housed in an Omni I/O breakout box),
b) Yamaha Motif MIDI via USB MIDI cable to computer USB in (not to the interface), and
c) The microphone transmitting the mechanical thump of when the key is struck.
RESULTS:
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The microphone thump audio was always printed about 2-4 msec earlier than the audio and the MIDI that travelled from the key through the Motif was.
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The MIDI note was always printed within 1-2 msec of the audio signal that travelled through the Motif - usually 1-2 msec after, but on one occasion 1 msec before, the Motif-generated audio.
So the MIDI from my synth arrives within 1-2 msec of the audio from my synth, and both arrive 2-4 mseconds after the signal down a mic cable does (the cable that measured the “thump”).