Also if you regenerate a section of track(s) RB now is trying to create the new generation (while reading the original designated PGMusic file and writing the new section) and ALSO writing the backup (or UNDO) file scrap so you can use the undo function. Again the drive is the most likely point of slowness if it is all happening on the same drive.
The above is being re-quoted by myself to point out that Undo is available in RB consistently, while in BiaB I don't think it needs to write an Undo .. you get what you get every time you hit play and it regenerates the song.
So there are 2 big differences in how generation is handled;
In BiaB it is faster (but less Undo options) which allows it to regenerate and play at the same time.
RB has more Undo capabilities because it writes an Undo scrap in the Temp Audio Directory (and generates what you selected) before starting playback. Which takes longer for obvious reasons.
Temp Audio Directory is a setting in Audio Prefs for RB. I've said this many times; it is worth noting! If possible use a different drive with little load.
I've crashed Audition, Sonar, ProTools, Reaper .. pretty much all of them. With proper setup (and horsepower) I've also ran all of them successfully.