Noel,
Well yes, midi will surely work faster...
But I need to hear how particular set of tracks sound when change is introduced:)

Maybe it is possible to start playing sequence while regeneration occurs? So it buffers first 5-10 seconds and starts playing while the rest is processed? For that, I think this has to change:

I believe, BIAB processes/renders tracks in Serial mode, in all theory, given the speed and multicore processing power of modern computers, it should process them in Parallel mode...