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it will cause performance issues when Biab needs to react in milliseconds to ad hoc chord changes - no pre-generation of anything

Reaper does that, it will smooth seek on the next beat or bar to the next chord anywhere within the wma source files instantly.
Most styles just have 4-5-6 tracks, the soloist is just playing in key/scale so it keeps playing the current riff without seeking chords, drums only change when part changes from A<>B. So the only chord seeking will be with the rhythm tracks.
All the data for each track in the style is loaded so on every chord change it knows where to go and how much if at all to transpose/stretch.
Biab was made how it is because of the old days, the old days have gone though there are still old users thinking the old way.
Pretty well all DAWs work this way by playing from any start position within the source files direct with non destructive editing.