Poor Diablo III: Reaper of Souls performance on a rMBP late 2013
I'm assuming you have the 15" rMBP with the NVidia GT 750M. If you're trying to run at native resolution, 2880 x 1800, with full effects, that card is likely inadequate.

The GT 750M I would estimate is roughly equivalent with a Radeon HD 6850, 6770, or even lower, placing it fairly low on this benchmark chart for the original Diablo III.  The frame-rates there get down into the 20's, and you have a) 25% more pixels then their highest tested resolution, and b) the expansion which likely has had graphical updates in the intervening two years.

If you want better frame-rate, you must trade off fidelity.  Lower the resolution, lower graphical settings until you find something you're happy with.