I've got a weird problem. I recently got a new Philips BDM4065UC UHD monitor for my Late 2013 15" retina MBP (it's amazing), connected it using DisplayPort 1.2 (also set on the display) and when using both the built-in display and the external screen, everything works amazing, the image is butter smooth at full resolution and 60Hz.
However, when I try to use my MacBook in closed clamshell mode (the screen is ridiculously huge, there's no point in using the built-in screen and there's not much space on my desk), it switches back to 30Hz. WTF?
I mean I would understand it the other way, but this just doesn't make sense. Is there some setting I'm missing? Since I'm not using the screen I'd like to use all the video memory for the external screen.