Who's fault is it that we now have a Biab Plugin, JJJ !
It was his original idea, he said "how about a new car", but I was the one to work out a way to make the engine as I could not see PG putting all the code into the actual VST then, so I said they could use the same engine RealBand users in the background.
Now we have come this far thanks to Adar's great programming skills,
I think it is now time to start moving the basic generate code into the Plugin, this can be done bit by bit for each function.
1st step I think would be rather than getting BBW4 to generate the track sections into RAM then render to wav, would be to just use the very first step of BBW4, the section data it decides to use from the RealTracks,
:Bar 1 = RT wav source file, time, length, pitch, stretch
:Bar 2 = RT wav source file, time, length, pitch, stretch
....
....
So the Plugin will just use this info to play direct from disk.
This will be a lot quicker to generate than generating even into RAM, so it will be quicker than Biab !
A wav will be rendered only if you DRAG, if not it will send the track playing direct from disk via the Audio Sends to the track in your DAW that will have the FX chain.
This way you can render the DAW project without the BB Plugin ever rendering a single wav.
BAND-IN-A-BOX VST DAW PLUGIN 2021.5 FOR WINDOWS/MAC®
Look Ma, no WAV!

You can try this if you have the wav version of RealTracks/Drums.
If you don't have wav just convert the 2 files from wma, m4a, aiff to wav first.
Open the Reaper project and point it to
bb\RealTracks\Guitar, Acoustic, Strumming Hank Sw 120\AcG011.wav
and
\bb\Drums\NashvilleSwing8^\NashvilleSwing8^_110_Style.wav

* remove .mp4 and open in Reaper:

Attached Files (Click to download or enlarge) (Only available when you are logged in)
RT 372 & NashvilleSwing Save (wav).rpp.mp4 (80.09 KB, 19 downloads)
Reaper-Play-RealTracks-From-Disk.png (128.89 KB, 146 downloads)