Custom ROMS recommendation

So i have the official 5.1 android and for 2 years i haven't changed from the official builds so i'm not familiar with custom Roms on nexus 7. Because the 5.1 is soooooo slow and the battery is very poor i want to ask you guys what custom rom should i install on my device and also what can i do about my battery. I don't want the newest OS, i want stability and performance (not for gaming, just usual stuff), and with a good battery life that would be great.
P.S: if you know a good ROM please send me a link :D.

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