How do you know this? Are you a coder? Have you looked at the code?
Yes, actually I work as a developer.
No, I haven't looked at the code, but having an application in 2013 with this powerful hardware and not being able to resize a window properly is signs of something not made very well!
Even the first iPhone with 1/100 the horsepower was smoother than this.
Well, you don't have to be a coder, just try resizing the App Store window to see the problem

There is no way this is the best Apple software engineers can do. In the nutshell: it looks as if they are recomputing/redrawing the layout way to often during the resize operation.
Yes, maybe the redrawing is a problem. But take a look at responsive websites that also are redrawn during window resize. They are much much more smoother.
And if they aren't good enough to make it run smooth, then just redraw it once the resize is done (like the iPad does when going from portrait to landscape mode). But I don't think that is acceptable in "the most powerful operating system" in 2013.
Resizing a window should simply just work! If it doesn't, the developers on Apple should work day and night to get that crap sorted out.