I have an old tablet - Asus Google Nexus 7 (2012) with 3G.
You can find some more specs. here - https://www.gsmarena.com/asus_google_nexus_7_cellular-5091.php
After receiving the last Android update the tablet became very unresponsive and slow. The battery drains for a day with no reason. I tried a lot of software to find out the reasons for that behavior, but without any success.
Over the internet I found a lot of users, actually all of them experience the same problem, and could not find any solution. The only way to fix it was to put the last 4* Android version - 4.4.4.
I thought I want to give it a try to see what will happen, because it is a shame such a nice device, with descent hardware, well made to be thrown to the garbage, just because it is not compatible with some software!!!!
All in all I followed this guy guide, without major issues:
It is difficult to say how nicely surprised was I, to find out that I have my old tablet back to business!!
It really worked fine again, no lag, responsive, my apps were OK, and they get updates. Amazing! The only problem was the nasty software update notification, which I could not remove. It wants me to install the sluggish Android again!? No! Never again! Not on this device!
Another amazing thing - the battery! After struggling for several years with many charge/discharge cycles, I though it was a goner.... Well it works like new!! For a week it drained only 50%!!!!! Of course it was without 3G and WiFi, but I updated my apps twice, and watched an YouTube clip.