Can’t tell if it’s any of those issues since don’t know what your system is. If you could re-post with System Info, that would be great. A simple test is to see how many video layers you can playback without rendering then that should be the same you should be able ot playback in Multiclip mode. When you post your sytem info, include your RT settings, have you taken advantage of the new Dynamic setting?
Thanks! Hope with info can help.