I think I accidentally found a fix for this last night. I have my default work space set to "color". That means the lumetri window is open. If I have it on "Creative" that mini-window with a still frame from the clip shows. When that is the case, the video will lag terribly in both the Program and Source windows. Changing the lumetri window to any of the other choices, i.e. Correction, Curves, etc. drastically reduces the lag on my computer to almost zero lag, which I'm sure is nowhere near as powerful as the computers you guys use.
When I set my quality to 1/4 I can actually play back smooth video and make adjustments while it's playing. Again, I only have a consumer computer with an Intel graphics card. Even if you're getting decent playback and scrubbing, try either closing the lumetri panel or just switching it to any tab other than "Creative" and you might see an even better playback experience.