Hi Zach, thank you for the fast reply!
It's a good sign that you don't see the lag.
On retina I tried Google Chrome - not as laggy (but still worse than the normal display), but Safari and Firefox are definitely laggy.
Indeed, I tried removing the black rectangles, but the performance issue is still there (it does get a tiny bit better, but going from min 4.90fps to min 6.31fps, which is still very low).
I recorded the performance with Firefox debugger. You can see on the screenshots below that the difference is minimal ...