Just tried the one MB, and was able to see the lag by pushing the apps to fullscreen. Am going to go back midweek when I have more time to look at one. Another member has also observed the same lag on 10.10.3 so it`s not isolated.
Lets see, am still open to one, equally much of my workflow is in fullscreen/spaces so this does concern as it wasn't just dropping the odd frame it was a pretty good slowdown. In another context this display machines get hammered all day, especially a new model in a busy flagship store.
Certainly inclined to hold off until people physically get them and we see the feedback.
Q-6
I didn't full-screen any apps, perhaps that's the difference. (I don't do it in real life, either, which is why I didn't try it yesterday on the store's rMBs.)