Nexus 7 Incredibly Slow
I have noticed the same issue on my Nexus 7 running stock Jelly Bean 4.2. I also noticed reboots. Do you happen to have HD Widgets installed? That seemed to be the cause of many of my slow downs, but unfortunately not all. Apps that use location services seem to be the issue with my slowdowns. Are you noticing the same thing?

This article goes into some of the issues seen on 4.2 that I experienced.