Android - Nexus 7 2012 is almost unusable after upgrading to the latest Lollipop. What should I do?

The official Google Developers site has the full instructions and image repository on how to install a factory image for any Nexus device, including your "nakasi" Nexus 7 2012. I recommend 4.4.4, but you are free to go back as far as you want until you're comfortable with it's use.

Cyanogenmod also has a stable 10.2 version (Android 4.3) - you can download the image file here. You can re-use the instructions found on the Google Developers site to install this as well (just target your CM unzip location instead).

BE AWARE: This will wipe your device, and this is all done under your own volition. Flashing a device - regardless of source or instruction - always comes with risk of damaging or rendering the device useless. We are not responsible for it!

Try 'JimTDI's Guide - Tuning the Nexus 7 with Android 5 Lollipop for Maximum Performance!' here https://goo.gl/X0HwAu The discussion for this post is here https://goo.gl/wqjv76

You can use the highly functional nexus root toolkit to do pretty much anything you need to on your Nexus device.

Nexus Root Toolkit

It allows rooting, return to stock, recovery and everything else you would need to repair your nexus 7 2012 WiFi aka 'grouper' / 'nakasi'. You can flash custom roms with it also.

Other tools include unlocking or locking the bootloader, adb tools, driver installs. .. you name it, it does it.

Works with all nexus devices released so far.

One of the most handy things about it is that it tells you what to do step by step...

Every nexus owner should have this as it's pretty much indispensable.