Poor Diablo III: Reaper of Souls performance on a rMBP late 2013 I just got a new rMBP (with dedicated video-card) and as a test I decided to run Diablo III: Reaper of Souls on Ultra settings.  It looks absolutely gorgeous but its 10-17 fps frame-rate is very disappointing. 

Are there any settings I can adjust to improve this performance? In Windows it can be "tuned" to max performance or battery saving; is there a similar setting in OS X?

Right now it's on AC power (plugged on the wall), Activity Monitor is reporting that the video-card is being used for D3, is there anything else that I'm forgetting to check?
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.