@jeff made some significant changes to the OpenGL Display Pipeline in Rhino 6 for Mac’s 6.20 Release Candidate. The original motivation was to fix:
RH-54092: Rhino doesn’t paint viewports if connected to Airplay (which is now fixed)
…but the changes went deeper than just addressing that problem. We suspect this may improve display performance 
. However, as with any fundamental change, we cannot test under all conditions and on all GPUs. Please be on the lookout for regressions and report any suspicious viewport display behavior by @mentioning @jeff and myself. Examples of regressions might be: blank viewports, thumbnails that don’t update, or extremely slow display performance that is not occuring in 6.19.
Please switch to the Release Candidate update feed and give it a try.