I use a rMBP 15" and it's fine in OS X on Mountain Lion from what I can tell. Yes, websites with complex layouts can lag on scrolling a little bit more than on a traditional MBP released the most recent years, but it's not something that makes me frustrated or angry. For example, Gmail, Facebook, Macrumors are all completely lag-free on Firefox on this machine. Silky smooth scrolling. But there are the few sites, usually heavily ad-ridden, where it can sometimes be noticed.I've heard the rMBP's are laggy because the GPU can't keep up pushing all those pixels. If the Pro can't handle it - how the heck is an Air gonna handle it?
And the MBA's already use the same GPU as the rMBP's use for OS X with default settings, so I think Apple already solved this problem with speed well enough although it's not a perfect diamond and just 90% there, so to speak.
I think the greater question (as I posted earlier here) is whether Apple will be able to meet the strict low power usage that people using Air's are used to by now, as well as weight, thickness, and price.