Yeah. Just look at this menu bar when switching focus from internal Retina display to external non-Retina ATD. http://cl.ly/2W172H0l3s0z This bugs the hell out of me.

Not so long ago I grabbed myself a G4 MDD with 768M of RAM — it was much more responsible in terms of UI. 3-4 browser tabs slowed it down though, while my current rMBP13" from 2012 works ok with a lot more.