Edit: Since this was migrated to the PC forum (thanks mod!) I'll amend this appropriately.
I'm having sluggishness on my windows 7 PC (Z440 qualified system) during multicam playback. Just 2 streams of 1080 XAVC Intra 100 media and marking in/out points is very very slow. Once the clips get into the timeline they seem to work just fine. Even when not using quad split the multigroup is very sluggish when marking in and out points in the source window. Playback itself is smooth- the video looks fine. Oddly, when no points are set the FIRST thing you enter is fine. So if you just enter an in point or just enter and out point it sets it fine with no stalling. But as soon as you complete the in-out it hangs up and takes 2 seconds to set the out point. I'm still on 2018.12.3 as well. I'm wondering if a newer version has fixed this issue.
Edit: disabling my hardware does not solve the problem. Another super stange quirk: If I zoom in on the timeline so as to only see approx 10 seconds or less of the clip, it marks in and outs fine. Zoom back out though and even marking a short 3 second moment causes delay. No waveforms, dup clips or other visual cues are on.
Edit 2: please let me know if I should cross-post this over to the windows forum