I have a super old 8GB Nexus 7 (2012) (grouper) tablet from Asus. It was a crime against computers that it was built with the USB port upside down.
For many years it has been unusably slow. I heard rumor it was that the ssd trim was jacked up and it would never work right. It had gotten so slow that it would take 10 seconds to register each touch event and was basically useless. Recently I had a go at fixing it and had some success. Here’s what I did. Follow these instructions at your own risk, they’re noted from memory and I haven’t been throught them step by step to make sure they super work still. I may be missing something. If you break your tablet it’s your own fault, not mine.
First I got adb working on my mac. Easiest way was:
brew cask install android-platform-tools
Turn on developer mode by tapping on the build number 7 times on the about device settings page. Turn on USB debugging
then I had to unlock the bootloader
adb reboot bootloader
fastboot oem unlock
Then install TWRP with
fastboot flash recovery twrp-3.1.1-0-grouper.img
Upload both of those to your tablet.
adb push krexus*.zip open_gapps*.zip /sdcard/
Reboot tablet in reovery mode (hold power and volume down buttons) press volume down until it says recovery and then press power.
Install from zip and select krexus and add zip and add gapps. then go (without automatic reboot) then clear the dalvik cache and then reboot.
BAM your nexus 7 should come up with android 6.0
NOTE: My domain requires encryption of android devices and I tried to encrypt it right away and it would just lock up even after leaving it all night. I found if I carried on withouth the encryption and set up my device a little bit. I turned on a lock screen and set a password to unlock it and then turned on encryption it worked fine.
My tablet is still not roaring fast but it’s usable. If it doing other things like downloading or installing apps it still grinds to a halt so I try and do only one thing at a time but it’s good to be able to use the kindle app again.
Hope you find this useful.