Could you please share with me more details about the issues you're facing when editing the rebar constraints? What is the specific behavior(s) that you would expect to improve and how?
Catching up on this, I'm updating the status of this idea. In Revit 2020.1 we released the "Rebar constraints editing performance improvement" which makes this process faster for shape-driven rebar in complex structural hosts or for bars in concrete host elements containing a large number of bars, such as a raft foundation.