@mcc My other theory is that this might be a consequence of having the jack audio thread crank the synthesizer, which in turn reads events off the ALSA input port, which means an intermittent batched processing thing is periodically polling a real time event source. However, I am under the impression that the audio thread wakes at a fixed cadence, and this wouldn't ever cause the song to get faster just compressed in weird ways, which is not what it sounds like.