@Troy:

You're welcome. I only have a 13" Retina MacBook Pro, hence it does not support 60 Hz—I didn't even bother trying to hook it up, although I guess I should at least try 30 Hz.

In my opinion using a modern display at 30 Hz is just not possible—everything behaves as if OS X is lagging, but it's just the display.

If you have a late 2013 15" rMBP (or newer) then you should be fine. I don't have one to test, but with the display closed (using only the external display) I doubt you'll see degraded performance, especially if you're using a discrete GPU instead of the on-board Intel Iris graphics.