To increase battery life, I have tried this, and it did seem to speed things up a bit...... Although whether battery life increased, not sure. However, here it is: Firstly you delete your email address in the systems section, then add it back in again. Apparently this stops it caching a huge amount of emails which it has been storing up since you put it on (or so the tip I read said). I also did this with some memory heavy apps I had such as Bloglovin, and Pinterest. All you do when you reinstall them is to put in your email address, and your password for the app, so keep these handy. You will also need your email password when you do the email install. Hope this helps.
I do think the new operating systems they keep having us install, eventually become too much for the old iPads. But mine is an iPad Air and is still going, and I am an iPad addict! ?