i think the difference is that BIAB starts playing before everything is generated. RB doesn't which i presume is just the way RB works. as it displays the complete wave form it needs to do everything before it starts. it's not the drawing, it's the generation.

when i cut and paste already generated audio it's instant but new material i agree takes a little longer but i've never thought it was a problem - just the way things are in RB as opposed to BIAB