I've submitted the RFEs.
I kind of figured that the HSL thing had something to do with the RGB -> HSL conversion, but I figured I'd mention it just in case there was something that could be done.
I did some experiments long ago and found that most of the slowness is caused by the renderer, not by the node tool itself. So I think our only hope is Cairo becoming drastically faster than it is and than our current renderer, and then switching to it.
I can't wait for Cairo to get fast, but I get the impression from reading the list that I may be waiting a while. Would it help things out at all if IS just drew a preview of the affected bezier segments as points were being dragged and waited to redraw the actual shape until after the mouse button was released (similar to what Illustrator does)? I've done some simple experiments and it seems that if I snip and break apart a small segment from a complex, choppily-updating path the small segments would update fairly smoothly as I tweaked them with the node tool. If that sounds promising (or if you don't quite understand what I am getting at) I could make some mock-ups of how I imagine it might look.
-Mike