Keep them on iOS10 for as long as you can. Consider doing the tvOS profile trick to prevent the update from auto-downloading. My 6 (with a lousy battery at the time) went from extremely usable to garbage as soon as I updated from 10 to 11. And since they stopped signing 10, there's no going back after you've updated. The new battery brings my 6 back to where it should be, speedwise. Which is slightly slower than how it performed under 10.