In my tests, I connected my Airpods to my iMac and selected them as playback device for BIAB. Even in the highest resolution (and the most 'sluggishness' of the UI), the chord display had NO delay! I don't know if this has to do with CoreAudio, but it seems like when using the AirPods, the audio driver reports its buffer size correctly unlike when using the internal iMac speakers.
Thanks. You have some interesting findings.
We would normally see the problem where the chords are displayed late when using a bluetooth speaker. It seems that something is compensating for the audio latency whether or not your AirPods are currently connected.
Does unpairing them fix the problem completely? (And rebooting Band-in-a-Box without them being paired / connected).