The Ole Nexus 7 (2012) is Great Again!
I've been playing with the 4.3 on my Nexus 7 for a couple of days now and I can confirm that all of the performance issues that many of us experienced with 4.2 have most definitely been solved.
Obviously, It's not perfect and there are times where Tegra 3 shows its limitations The new Maps app isn't all that smooth when panning around for example, but outside of that, everything is pretty much back to the way it first was when this thing launched with 4.1.
As far how Google fixed this; I'm guessing that the update to OpenGL might have had something to do with it. Although, I think the most likely scenario is Google did something to memory management. Generally, its seems like 4.3 is using anywhere from 200-400 mb less of RAM. I'm not sure if that new management is something specific in 4.3 for all devices or just a specific change for the Nexus 7. That's something I would be really interested in finding out.
But regardless, the bottom line is that the Nexus 7 2012 is a great tablet once again.