After upgrading my Nexus 7 from Android 4 to 5.1, it is so slow as to be unusuable. What can I do to make it usable again? I've seen reports here and there online where users have reported similar frustrations. Has anyone had success fixing whatever issue is crippling the tablet after upgrading to Android 5 (Lollipop)?
I did a factory data reset and everything is running much more smoothly:

Settings > Backup and Reset > Factory Data Reset

I had to log back in and reinstall my apps. This wasn't too onerous in my case since my tablet usage is limited mainly to the Amazon Kindle and Pandora apps. 

For a more nuanced approach, see this Tuning Guide for the Nexus 7.