Currently owning rMBP and having updated to ML no didn't totally resolve issues (better). The GT650 is taxed at times and think next year they upgrade the video card again with (internal and external) to help performance. Because IMO it is alittle underpower now. And like I said before just on Facebook or other CNN GT 650M is getting used. And when battery gets below 5% it only uses the internal video card to save battery life and machine becomes dog slow. So yea MBP 13' WILL need a discrete video card. And don't think coming this year. Sorry dreamers..
It is a software problem. 2880x1800 pixels is nothing to modern GPUs. Redrawing every pixels on each screen refresh means the GPU needs to be able to process 311,040,000 pixels per second. Sounds like a lot uh ? If we look at nVidia chips, the first one to reach about 300 million pixels per second was the Riva TNT 2 Ultra :
http://en.wikipedia.org/wiki/Comparison_of_Nvidia_graphics_processing_units#Pre-GeForce
Yep, that's before the GeForce series. That's a 90s GPU, before we even called them GPUs (the popular term back then was 3D accelerator).
The problem isn't the actual pixel count, that's nothing once the framebuffer is built, it's just one big copy operation. The problem is probably that Apple does multiple pass rendering of its UI with too many memory read/write operations instead of maximising buffer passes. That's the big problem with modern GPUs, they are plenty fast to push the pixels, it's feeding them the information to do it with (all the layers to blend together, the compositing effect) that slows them down.
The Intel HD graphics is probably just as able as the GT650M (the reason it kicks in on Facebook and some other sites is the use of HTML 5 Canvas/Flash and other graphic "intensive" operations. Webkit enables the dedicated GPU for that), Apple just needs to optimize the pipeline better. Why you're seeing better performance out of the dedicated GPU is basically just brute forcing an inefficient rendering pipeline through, and has nothing to do with fill rate.
Also, when getting to 5% battery, it's not just the dedicated GPU that gets turned off, your CPU also probably gets throttled through Intel Speedstep technology.
My '08 uMB 13" (the '08 13 MBP! @haushinka : haha, kidding!) was able to drive both its internal 1280x800 and an external 30" ACD at 2560x1600. That's very close to the same pixel count : 307,200,000 pixels with a 60 hz refresh. It did so on a 9400m nVidia IGP.
