Most efficient OS for a 2012 Nexus 7?

[AC Question]

I have an older 2012 model Nexus 7, Model ME370T. Earlier this year I let it update to Lollipop and it became so sluggish it was almost useless. I'm sure the relatively slow processor and lack of memory my older unit has was the issue.
Anyway, I'm running Jelly Bean 4.2.2 on it right now and everything is fine. I was wondering, can someone suggest the most current OS I should run on it that will give me the best features without slowing it down too much?

Thanks.