I'm sure I could google it, but I'm curious what is laggy? UI? Maybe it's a graphics issue. The rMBP has pretty amazing graphics capabilities so that might be it. I'm just using a USB 3 HD for audio. SSD for system.
For me (and for many users) it occurs when opening a plugin that has dynamic visualization, for example Channel EQ. I am unable to open two and sometimes even one instance of Channel EQ with analyzation enabled without the GUI lagging for anywhere from 10 seconds to 2 minutes. I can hit the space bar, click stop on the screen, try to manipulate application controls - nothing happens. It just keeeeeeps playing. And then it will go through and do all the things I tried all in the course of a split second. It doesn't happen consistently. I have been working on a new track for the couple of weeks. Some days I have tons of lag issues. Some days, none at all. This is the problem Apple are having as well, and why it's taking so long for them to solve the problem - they can't recreate it consistently enough! But apparently it's due to Core Audio issues. I don't really understand what that means, so take that with a big old dose of salt!