Thanks for all your help and patience with this. Rhino 6 for Mac 6.17 Release Candidate 4 should not exhibit this freezing. Please let us know ASAP if you do indeed get the same sort of freeze.
Gory details: Unfortunately, we had to downgrade the embedded Mono version in order to avoid this freeze in the 6.17. We hope to work-around this in a subsequent update so that we can update Mono and get the benefits of those updates in terms of performance and stability.