If I try to open a frozen Biab SGU with all mixer tracks in RB it loads the BB tracks and all the Utility track's RealCharts midi but not the Utility track's audio.
If i change the song tempo it has to re-encode all the wav files to the new tempo, it would take even longer if it loaded the Utility audio in also.
Reaper just instantly loads all the tracks in without creating wav files, and instantly changes the tempo.

See Video: Biab-Frozen-Tracks-RB.mp4