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.
User vocoderism published a pure ASOP variant of the Android M Firmware on XDA-Developer for the first Nexus 7 (Codename: Grouper).
Thread: [ROM] 6.0 Android MarshMallow Pure AOSP for Google Nexus 7 2012 WIFI
From this point on you proceed at your on risk. Warranty will be void flashing a custom Rom.
Each link leads to the needed file for each step. Download the files prior to step one and store the files on your tablet. This article doesn’t cover the root process.
- Boot into recovery (Power + Vol. down choose recovery and press Power button)
- Flash recovery image TWRP 22.214.171.124
- Reboot into the new recovery
- Perform a factory reset (Wipe)
- Flash 6.0_Grouper-MM.zip ( Download 6.0_Grouper-MM.zip / Build 10/31 )
- Flash Google apps ( Gapps 6.0 )
- flash root Direct Download (optional)
- Reboot and setup your device.
The first couple of minutes the tablet may feel a bit slow. But after all background processes finished the gain in performance is quite noticable. Also the battery drain is reduced.
Changed download link to new 10/31 Build. Issues with frozen device after deep sleep seem to be gone now. Auto-rotation problems still exists.