We believe we may have fixed this issue with older Macs in the latest 6.21 Release Candidate for Rhino 6 for Mac.
We have not been able to do testing on all the Mac models that were reporting this issue, but we did confirm the fix on at least one model.
If you were unable to use Rhino 6 for Mac before, please test again with the latest Release Candidate and you might be in luck.