Nexus 7 2012 is almost unusable after upgrading to the latest Lollipop. What should I do? I have a Nexus 7 2012 wifi. With the latest version of Lollipop it has really become unusable, freezing for a minute or more, or even rebooting while loading a page in Chrome.

I did a system reset, but that did not change much past the few first minutes. 

What is the recommended version to have again a usable tablet? Is anyone having a "runbook for dummies" to install Android 4.x (or CM or whatever works fine) ? 

Most of the tutorials I found on the Internet are for geeks wishing to experiment. I just want a running system with minimal fuss/risk.
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!