How can I maintain stable/smooth performance permanently on a Nexus 7 2012 (wifi) [grouper/nakasi]? After a day to a few days, the n7's performance will degrade sharply to the point where a reboot is necessary for the device to function properly.

I've tried numerous ROMs, a few kernels, tweaking CPU governors (mainly just to performance) yet no matter what I try it will work great for a while but if I let it sit idle for a day or so load times for doing anything jump up to unacceptably slow levels.

By unacceptably slow I mean on the order of magnitude of 10 times longer than normal or more. UI elements and the UI itself are often unresponsive to very slow and even when it catches up to the first command (ie home button), the next command still takes just as long (ie loading an app).

This is a single user device and has very few apps and fewer running. There is always lots of free ram usually 50% or so.

Right now it's running lollipop (ART) but no difference.

Is there something I can do so I don't have to reboot every day?

How can I diagnose what component(s) specifically need the reboot to begin functioning normally again?
I set up auto fstrim with Device Control [root] and after a few weeks have not seen the same degradation I was seeing before. I am running CM12 lollipop nightlies.

You can set it up in Extras >Filesystem :