I increased the buffer size to 1000 and there's no delay before playback. It continues to play immediately. Not sure why this would be, but could be the problem. I've also noticed a note will sometimes play at the specified negative delay, but when the playhead reaches the note, it will trigger the note again (as if there are two notes, when there's just one). The problems I'm having are intermittent and change randomly, so it's hard to pin down exactly what the solution could be.