Based on my current, admittedly short, experience, I'm blaming Google.
I recently completely reset/reformatted my 2012 Nexus 7 and put CyanogenMod's CM12 ("Lollipop") nightly on it. I intentionally did NOT install the "gapps" (Google apps) add-on.
So far, my own 2012 Nexus 7 has been working great, better even than it was with CM11 ("Key Lime Pie"/Android 4.4.4) with all the Google bloat.
Google has been shoving more and more of the "Android" experience into their apps instead of the OS. The not only are the "apps" and "services" getting more digitally obese, but there seem to be more and more of them every release, just loading up and clogging up ram and occasionally "updating" themselves online doing who-knows-what.
I feel like I saw similar (though less obvious) improvements in performance with previous now-"obsolete" devices that I've similarly purged and custom-ROMmed without the Google Search/Play/Music/Plus/News-And-Weather/Mail/Now/etc.
You're kind of stuck with it if you're dependent on apps that are only available from the Google Play store, but I'm finding I can get everything I need from f-droid instead, or through the web browser, at least so far (and for my own needs).
Anyway, point is, so far it doesn't seem to me like it's really "Lollipop" that the 2012 N7 has a problem with...