i have found the one thing really slowing down rhino in bigger cad files from autocad for instance are blocks only, once i hide those the performance normalises. at least in some 800-1000 sm projects. making it a real pain to work with till hidden. the problem is of course that blocks usually contain important information, and exploding them often if not always ends up in messing up the entire data inside the blocks. maybe that is my fault and i had some scaling issues idk.
anyway Rhino 5 for mac was very fast, in these regards
@stevebaer recommended trying the testmetal command which i did not get to, i still have no mac which is able to run v8. i wonder if that improves blocks?