What is the best Android OS to rollback my Nexus 7 (2012 WiFi) to?

[AC Question]

When I bought it my Nexus 7 was great. But with the latest several updates to the OS it has gotten progressively slower and with 5.1.1 it is currently too irritating to use. My assumption is that this device doesn't have the CPU power to run the newer OS efficiently. I'm thinking about flashing an older version of the OS onto the device.
So 2 questions:
Is this a workable fix (good idea) to speed up my Nexus?
What is the most stable version of the older versions to flash? I'm thinking 4.4.2 but I could be way off base.
Thanks.

 

[Rukbat]

I run 4.4.2, because it's the latest version I can get rooted and get Xposed to run on. It's been stable for me since it came out. (I've tried Lollipop on a phone that can handle it. I'm still not convinced that a memory leak (which wasn't fixed until 5.1) is a good thing to have. (But I've only been designing system software for 42 years, so I could be wrong.)