Because vocoderism seemed to stopped working on his Android M Grouper firmware and the autorotate bug was still an issue I had a look for another Android Marshmallow firmware that worked well on the Nexus 7 (2012). I found a Pure Nexus version for the Nexus 7 that worked quite well.
Tag: Android M
In my blog post from Sep. 2015 (Speeding up Nexus 7 (2012) Tablet after Lollipop upgrade) I evaluated several options to get the Nexus up to speed again even with Android 5 (Lollipop).
But even with the changes in place I still got a lot of lag and unresponsive apps. I considered a downgrade to 4.4 when I noticed that there is a Android M Firmware for the Nexus 7 on XDA-Developers.