Google Currents may be causing lag on Nexus 7 running Android 4.2
Some users have been complaining of pretty bad lag on their Nexus 7s ever since updating to Android 4.2, and there is an unofficial fix if this problem is happening with you. Apparently, some people are finding that the lag issue may be caused by Google Currents, but this can't be 100% confirmed because neither ASUS nor Google has acknowledged that there is a problem.
You shouldn't be surprised that the diagnosis and fix both come from the great community over at XDA, where it seems that users have found that disabling background syncing for Google Currents should clear up any lag issues that you're having on your Nexus 7. So, if you're having the problem, just pop open Google Currents, tap the menu button (the vertical ellipsis), go to settings, and uncheck the "Enable background sync" option. Alternatively, you can go to your device settings, and the Google account settings to disable Google Currents sync.