I have had my rMB and my Core i7 13' MBPr side-by-side on my desk for the last couple of days to compare and see if I was going to keep my rMB or return it. For the way I use my computer, I can't see much of a noticeable difference in performance. Apps open slightly faster on the MBPr, but it's hardly noticeable. I'll take the noticeable portability difference any day when compared to the negligible (perceived) performance difference.
Those are my experiences, and everyone's are going to be different. I don't get caught up in benchmark scores, or measure the time (or bounces) it takes to open an app. I use a computer the way I use a computer. And given the way I use my computer, this thing is rock solid.
Lightweights aren't all bad. Cheap dates, for sure. hehe... ;-)