What types of programs do you plan to run on it? I'm sure if I wanted to try to find a lag issue and ran a battery of tests with several different Mac and PC models side by side I could detect one, but in everyday usage I don't notice it. I do notice how sharp the text is, and how detailed photographs and images are.
I had an 11" MacBook Air previously. The 13" rMBP is a lot nicer, even if my 2.0GHz i7-equipped Air offered similar overall performance. The color gamut is richer on the rMBP, and the available resolution is much nicer. I run it in "Best for Retina" most of the time, but being able to emulate 1650x1080 or 1440x900 is nice. With QuickRes a whole host of other resolutions are also just a click of the mouse away. The slightly longer key travel takes some getting used to, as does the extra 1.2lbs, but the transition is pretty easy.