The enormously slower than rMBP mid-2010 13" shows almost imperceptible lag. The rMBP should theoretically drive it's display with no problems unless there is an inherent inefficiency with the way it runs "virtual" resolutions.
The “inherent inefficiency” is the fact that it is rendering almost 3× as many pixels—your display is 3.7MP, whereas the Retina MacBook Pro is rendering 9.2MP at the 1920×1200 (HiDPI) setting.
Not only does it have to render that much more, it then has to perform complex scaling algorithms (it can be done quickly, but look bad) to downsample the resulting 3840×2400 image to 2880×1440.
It is most likely to be the downsampling stage that is causing the loss of performance, but it could well be that these processing demands at 3840×2400 are simply beyond the limits of the GPU—not necessarily its processing power, but its available bandwidth. Or it could be something else entirely.
With regards to downscaling, I tried the 13" with cinema display combo at 2048*1152 and 1600*1200 letter-boxed and there is definitely a touch more lag.
That is
upscaling, not
downscaling. Generally this is handled by the display hardware, not the GPU. (though it
can be done on the GPU) and is less taxing than downscaling.
When you are upscaling the image, all the rendering is done at a low resolution (less taxing) and then the final result is scaled up.
When downscaling, the rendering is done at a very high resolution (very taxing) and the result is scaled down.
Still, scaling should be cheap compared to rendering. GPUs (even the HD 4000) are very good at texture filtering and have plenty of bandwidth for these kinds of tasks.
Of course, it is possible that the very high-res texture cause some problems with the cache which degrades the performance... when I get my rMBP I will try to run some test with OpenGL and large textures to see how is the performance.
The type of image scaling that is done in 3D rendering is generally very poor quality—not at all suitable for something with text etc. and it’s my understanding that Apple have written their own scaling routines so that the results are consistent between graphics cards.
