If you get the Windows Task Manager up and look at the RAM being used by bbw.exe you will see it grow as the tracks are generated, add double the amount of bars and double the RAM is used.
The saved SGU with frozen tracks will just contain the the position, length and transposed information only of the section from the source wav/wma so it will use that information to load the same section back to RAM rather than a new random one.
In RealBand it uses bbw2.exe in the background to generate up the tracks direct to wav in a temp folder then loads them into the RealBand tracks as wav. So it will use all 7 tracks in bbw2.exe to generate up 7 MultiRiff of the same intrument to wav segments in the Realband temp folder then overwrite that section of track in RealBand with the MultiRiff you choose.