Have you seen the video? Try yourself
13" 2011' MBP here:
- Right after App Store startup: buttery smooth
- While it is loading/laying out the screen contents: laggy just like yours
- After it's fully loaded: much better than yours but not buttery smooth
You're complaining about something that is inherently slow everywhere: resizing complex view layouts. The App Store app is a web view (embedded Safari view) and Safari doesn't handle resizing well.
To be honest, I think you're focusing on the wrong thing. Overall performance in OS X is stellar for most things, but a few do suffer (window resizing has kind of always been worse than on Windows due to its architecture, but window dragging is much better and in-window animations are much better).
OS X still uses the CPU to draw a LOT of on-screen elements and the GPU to composite them. That's why window dragging is great it's just moving a pre-composited square onscreen using just the GPU and window resizing is slow it's re-rendering it partially on CPU, and then sending the result to GPU. This is one of the reasons why I haven't been interested in the Retina Macs yet: their CPU isn't much more powerful than the pre-Retina models, but the pixel density quadrupled. Their GPU is powerful enough to push these pixels, but since OS X kind of still relies on the CPU for some things, sometimes it'll get CPU-bound.
I could go on for a while about how this CPU/GPU duality affects performance of certain on-screen operations, but I don't think it's worth it right now. If you really want to know, feel free to ask.
Bottom line: the Mac you bought is a great and powerful machine. The low FPS you saw in that particular case is because you went straight for OS X's Windowing-system achilles heel. I wouldn't return it.