Hi Brian -
Our editors are working in Premiere CC2017.1.2 with 6K Red Raw but usually at 1/4 playback resolution to avoid above symptoms you mentioned (same Mac model). If they need full playback for a big review or the like I recommend rendering out their sequences beforehand whenever possible.
Like Ronald said earlier Premiere versions (CC, CC2015) were semi-nightmarish at times but it's been much more solid for us in later iterations. While our main editors don't work with proxies (they don't want the added workflow overhead), we have an independent filmmaker who shoots 4k ProRes and she and her assist. editor have been using a proxy workflow with great success since CC2017. If you dial in your import settings and they're using shared storage and the source files never get moved then the offline/ online process can be pretty seemless. Plus it can solve many of your playback related issues...
That being said, other Premiere playback issues we've experienced have been due to misconfigured BlackMagic cards, SAN/ storage issues, misconfigured caches/ scratch disks/ database, people plugging things into the wrong TB bus, cheesegrater users selecting OpenCL for projects and trashcan users selecting CUDA, etc. etc. the list goes on.
Also here's some useful tests on getting creative with Premiere's caches/ scratch disks:
Hope that helps!