I am not sure how I feel about this solution. The phone always runs in power saving mode (the processor speed part of power save mode). It can also be seen from how enabling power save mode, no longer changes the benchmark scores. The lower the battery, the slower the phone becomes. This has been confirmed via benchmarks, which now run at about 40-60% of the speed from November last year. So for me, iOS 10.2.1 works better in that there are less shutdowns, but I have to accept that everything now runs at about half the speed, compared to iOS 10.1. It is these voltage drops that can cause unexpected shutdowns. Then it throttles down the processor speed to avoid power spikes that would otherwise have caused drops in battery voltage. How it works is that it somehow detects that I have a questionable battery, either via serial number or measurements. I believe the problem is caused by Apple's battery 'fix' that came in iOS 10.2.1. Everything seems to run and start slower. There is no difference when I enable low power mode. I get about half the benchmark score as I did with iOS 10.1. Sorry, I cannot help you, but my iPhone 6 (not 6S) has the exact same problem.