My nexus 7 was lagging too, it was after i installed this one app. I uninstalled the app and it would still lag but only on the homescreen. I removed the play store media widget and the lag disappeared. I have about 8 widgets (constantly refreshing) and a live wallpaper and still lag free. If you still have that default media widget, try removing it.
Though I still see lag, mostly when im downloading apps or they're installing but once the app is done downloading/installiung the lag goes away. This same issue happens on pretty much every android device i've ever owned, even happens on my iphone so not a big deal.
Phone: Apple iPhone 4s 64GB White
Tablet: Asus Nexus 7
Rom: Stock 4.1 Jelly Bean