Does someone really know how OS X handle Retina ?

I read as many review as I can to understand how OS X handle Retina. It seems the system draw a 4 times bigger image (ie 3360x2100 for 1680x1050) and than resize it to 2880x1800 to send it in the frame buffer. This is what Super Sampling Anti Aliasing do in 3D graphics card for gaming.

What is not really clear with Apple, is who makes each part of the job ?

According to what I read, CPU handles most part : high resolution image calculation and resize. Next, the good sized image is send to the GPU and than to the screen. So, if this it's true, lag comes from CPU not GPU (bandwidth between RAM and CPU is big enough, HD Graphics fill rate is suffisant, 512 MB even with shared memory is far more than requested for 2880x1800).

I also noticed when scrolling on heavy website using HD Graphics 4000 or GeForce GT 650m that CPU usage is maximum 12,5%. I think only one thread is dedicated for image processing. It's not a too big job for a Quad Core with HT, but one thread for a Core i5 or i7 only Dual Core even with HT will takes as much as 25% of the CPU. This leads to more laggy scroll.

PS : Sorry for my poor english. I do my best to try to explain what I think about rendering and what cause the lag. I really would like to know how it works (not only according to my understandings).

Thanks Purant. I remember Anand Tech review. This is the most complete and trusty one I read about. But it doesn't explain why lag is about the same when I use the HD Graphics 4000 or GeForce GT 650m. Also Anand Tech review was done with Lion and Mountain Lion brings big performance improvement (by doing it on the CPU ?).

Also I read (but did not remember where) a guy telling that Apple use to do scaling on CPU rather than on GPU because using different GPU (iGPU or dGPU) may lead to some different image quality due to internal teaks used by Intel and NVIDIA to speed up such kind of task. In other words, switching from iGPU to dGPU may be noticeable due to visual differences (at switching time).

I'm not a developer but I have some knowledge in hardware computer. According to me, what he tells looks true (at least for 3D rendering in games).

So I'am aware to any tech info !