That is true. I have a 15 retina and I do experience the graphic lag, mind you it is not ideal, though apple has improved it since it first launched . And to be honest i forgot about the lag, as I had huge issues with the wifi for a long time. All working fine now, but it took quite a few software updates to sort .
I had a 15" rMBP prior to my 13". It's noticable worse on my 13", to the point where I thought about going back to the 15". But at the end of the day portability is more important.
As much as Apple may have improved this in software, I feel this is primarily a an issue w/ the IGP playing catch-up to the needs of HiDPI scaling. Every new Intel chip seems to promise some 30-40% improvement so it will eventually be a non-issue, but right now I just don't think the graphics horsepower is there.
I'm here defending the rMB because TBH I was expecting it to be worse than this... to feel that it's in the same league as the 13" a refreshing surprise. At the same time I can understand how anyone coming for instance coming from an MBA would be underwhelmed by it.