I think I have finally solved the performance problems I have been having with my ASUS Nexus 7 tablet. It's easy to find reports of problems with first generation Nexus tablets. A google search quickly reveals that lots of people have been having problems with these tablets for months. Upgrades to new versions of the Android OS don't solve the problem, in fact, they often seem to make the problem worse. That's certainly been my experience.
I got my tablet about 2 years ago. It worked beautifully at first, but over time got slower and slower. A few months ago I was hopeful that an upgrade to Android 4 would help. When it didn't really solve my problems, I was able to manage the problem using an app called "The Cleaner." By running this app frequently to free up storage and memory, I was able to use the tablet to read email, news, and stream Netflix to my Chromecast. But I would often have the same problems that others report--long delays when launching apps, frequent pop-ups reporting non-responsive apps, and slow, sometimes unusable performance from apps.
Then a few days ago the upgrade to Android 5.0 came and again I was hopeful that it would solve my problems. It only made them worse. Once I upgraded to 5.0, the tablet was essentially useless: very long delays in launching apps; the screen would go blank and the tablet would be unresponsive for several minutes. But after cleaning up a few things on the tablet, it seems to be working now, almost as good as new. Here's what I'd recommend you try to get going again with your Nexus 7.
If you can get the tablet running (be patient with it and just wait) you should launch the settings app. When you select the the "Apps" section of the settings, you'll eventually be able to see a list of all the downloaded and running apps. Switch to the list of "running" apps. Now you should see (I'm assuming you have upgraded to 5.0) an apps screen like the one shown here. By adding the amounts listed for System, Apps, and Free memory you can see that the device has 1 GB of memory. And you can see how much memory the currently running apps are using. In my case, the running apps were using too much memory. You can see from the list how much each app is using. There were several apps running and using memory that I really don't use. So I deleted those from the tablet. Once I got the memory in use down below 384 MB my tablet performance improve dramatically. Then when I deleted a couple more apps and got the memory in use down to 234 the performance was back to what I had when the tablet was new.