Graphics performance - external monitor

Question

Hello everybody.
I've run into a problem..
When I add an external monitor to my rMBP 15", the graphics performance is reduced by quite a bit. Example: when I play League of Legends on my built in monitor, it runs smoothly at 60fps with almost the highest graphic settings. But when I play it on the external monitor, with the lid closed, it's going at 30fps with the lowest graphic settings.
Here's the twist..
I bootcamped it, just to test if there'd be any difference. And surely, there was. Suddenly I could play with the highest graphic settings at 60fps on my external monitor.
Here's my setup:
MacBook Pro Retina 15", mid 2012.
Asus VE278Q
The monitor is connected with a HDMI cable.

Answer 1

I have the same hardware, a rMBP with the quad 2.6, 16 gigs of ram and the NVidia 650.  I don't have the problems you are describing but I play different games - mostly borderlands, so perhaps that makes a difference.
The first thing I would suggest is to use something other than an HDMI cable.  I can't remember the details but I recall reading that the throughput of the HDMI is limited, especially compared to thunderbolt, so I would try switching first.  Of course you are roughly quadripling the pixels you are pushing when you go from 1440x900 (native retina) to 2880x1800 .... so that could be it, and I would also try to scale the external monitor 1440x900 ... and see what happens.
Lastly, most games run a lot faster under windows than OSX.  Sad but true.  I sometimes run the same game on my server ( a proliant with a Xeon quad @ 3.3 ghz with either a decidedly low end GT630 video card.  It smokes the retina MBP, badly for whichever game I have ever tried. They look better, they run much faster.  That said, I still like to game on the Mac and only use the Server when I need a third machine when my daughter comes to kick my butt.  I personally will take the ease and elegance over the speed, since the rMBP is fast enough.