"I always thought it was just the pure polygon count. If the morph brush is faster on plain meshes than subdivided ones, that would be useful to know."

I did a little test with my V4 "Zombie" morph.

V4 has 76181 polygons.

Morphing her "as is" with the Morphbrush is not a big problem.

But at SubD 1, everything turns very slow.

I exported her then at SubD 1 and imported her back as a welded prop  which now had 264096 polygons.

Morphing that prop wasn't as effortless as morphing the unsubdivided V4 figure of course, but still noticeable easier than morphing the subdivided V4. Much less lag while brushing, less jerkyness, better smoothing.

Sorry, I can't help with the implementation. I don't speak "code". Just some practical observations here. :-)

But Poser already CAN transfer morphs between similar shaped figures that have wildy different mesh topology, and it can export a subdivided mesh as a prop. (You can for example transfer M2's muscle morphs over to M3, and if you use the M3toM2 figure, which features the M2 shape [But not the actual mesh], then the transfer is almost perfect)

So it seems that there is the capeability to "freeze" the subdivision inside Poser.

And then the "high res" morph might somehow be able to be transferred from the subdivided prop to the subdivided figure it was originally derived from.

If one could really manage to transfer a hi-res morph from a prop to a subdivided figure, this means that unecessary bodyparts could be left out.

In my example, I would only export the torso as a prop to work on the ribcage, freeing even more computer power for operating the morphbrush.