Because Android 5.0 Lollipop works pretty slow on the Nexus 7 (2012) tablets, it is recommended to wipe the cache partition once a week.
It takes like 5 minutes, so here is how you do it.
1. Shut down the tablet.
2. Press the Power button + the Volume down button (at the same time) and keep pressing them until the screen starts.
3. Press Volume down until you see “Recovery Mode”.
4. Press the Power button (to select).
5. The Google logo should appear and after around 10 seconds, the Android logo with the front panel open, saying “No command”.
6. Press the Power button + the Volume up button (at the same time) and keep pressing them until the screen starts.
7. Press the Volume down until you reach “wipe cache partition”.
8. Press the Power button to execute the command.
9. Press the Power button to reboot the device (reboot system now).
After your Nexus 7 starts, it will work somehow slow for about 5 minutes. After that, it should run very smoothly, like you it is running on the old 4.4 Kitkat. But remember, it will get slow after a few days and you will have to repeat the process.
Android 5.1 wasn’t released yet. It is possible that the new version will fix the lag on the Nexus 7 (2012).