On a hunch, while coding up and trying to optimize the multi-stage analysis from the hybrid stitching thread, I tried to do the fluid.bufmelband~ part of the patch on just 256 samples, and starting that process before the larger 512 analysis window finished thinking that the 4-7ms it took to create and process the IRs could happen while already waiting for the rest of the descriptor analysis to happen.
Sadly, I was disappointed. I played with a few analysis window and fft settings, and it looks like 512 samples with 256/64 for fft settings is as low as I can go (with this approach) while still having useful envelopes being produced. I would either lose some of the high end resolution, or get weird lumpy bass stuff.
So for now, doing it with HIRT is the way to go until (hopefully!) @a.harker has some magic Framelib-based filter/IR tools.