You'll be pleased to hear Photoflow is massively faster (on my box). It responds effectively instantly when moving a point on a curve (guess <= .25s?). Same with the basic adjustments like contrast, very fast. And it's not that there's a big pause when you "ok" back to Gimp, that was quick too.
I set 32bit f.point, like previously, before going into PF. PF was using all cores for the basic edits, and I think for the curves too, can't exactly remember. I didn't try layers.
If I was being picky, I'd suggest make the curve points behave like the basic edit sliders in the sense that the latter update the image preview as you drag with the mouse button held down. The curve change isn't seen until you release the mouse.
I will probably explore PF further. RT does a lot. When I need Gimp (or PF?!) it will usually be to do a few local adjustments, so I mainly need lasso, feather selection, put in new layer, apply changes. I'm guessing PF will do that. But having said that, I do want to explore CCE and the Gimp linear processing that's currently being developed, in due course.