Any idea what could be preventing me from getting the same result?
At first glance the main difference between MX6 and MX5 youtube playback is that in MX6 the codec used is VP9 (vp09), while in MX5 it's AVC (avc1). I know that VP9 should be better, and it allows resolutions over 1080p on youtube, but for some reason it does not work properly in MX6 (but works fine in Chrome).
Another thing I noticed while looking at stats is that it doesn't seem to be a bandwidth problem, but a playback one (maybe hardware acceleration?). My system is Threadripper 1950x / RTX2070 with the latest drivers.
These are the stats of the same video (paused) in Chrome (no dropped frames) and MX6 (over 1/3 frames dropped):