CyanogenMod on a Nexus 7 2012 is usable! Yes! Even though the tablet is over 3 years old it runs!!! Very well. I will stick with Android KitKat though. I have overclocked my Nexus 7 to 1.6 GHz (Stock is 1.3 GHz) The nexus 7 can be overclock to 2.00 GHz if you really want to cook it. I also overclocked the GPU to 520 MHz. The performance really impoverished. Gameplay is much better when it is over clocked. Battery life is definitely a problem if you overclock it though. Especially with an old worn out battery. Android 5.1 was pretty lag free but if you download all of your apps it gets laggy. It is really cool to play around with but I think I will stick with KitKat. As much as I like 5.1, I need the tablet for reading and school and it is not very stable with all the apps I need. It can watch YouTube and stream videos just fine. Playing games felt the same as KitKat but loading the games took longer.
You Can Check Out This Video For Living Proof Of It Running: