My old Nexus 7 tablet from 2012 doesn't support the latest Android release. Instead of installing custom firmware from some independent developers, I decided to go back to the smoothest Android version I can find. Asus skimped on quality storage and Nexus 7 was pretty slow and laggy, especially when using Android 5.x.
Google has a page with factory images for Nexus devices and it's pretty easy to install any Android version that's officially available. You have to backup your data, enable USB debugging and run some scripts.
I've installed Android 4.4.4 and Nexus 7 was much smoother, but there was still room for improvement. Android 4.3 was even better and I decided to keep it. It's like having a completely new device, even if it runs some outdated software released 2 years ago.
It's difficult to optimize new software for old hardware, especially if manufacturers don't care about quality, cut costs and ship poorly made devices with obvious design flaws. On there other hand, Google has its own issues with software optimization, memory leaks, battery draining software and other bugs. When properly optimised, Android runs well and users are happy, but this doesn't happen often. With so many devices to update, manufacturers and even Google take shortcuts when it comes to old phones and tablets. Some stop updating them, others release unfinished software, hoping to encourage users to buy new hardware, while others spend more time improving the software for the latest flagships.
Thankfully, you can downgrade and go back to a software that actually works well. Apple devices rarely allow you to downgrade and usually for a limited time, so you're stuck with phones and tablets that are suddenly slow, laggy and crashy.
This post was written on my Nexus 7 running Android 4.3.