Quick fix (if Noel is on the right track here)
Use Edit - Track - Make All Tracks Regular Tracks.

Doing this causes RB to not generate everything every time. When you have 'BiaB tracks', RB regenerates every time you hit play (like BiaB) but, as Noel mentioned, it generates it all before playing and thus takes longer.

At what point do really notice the lag?


Make your sound your own!
.. I do not work here, but the benefits are still awesome