What we do know—and we have to say we know this only from Primate Labs, because Apple has not released any of the technicalities you're speaking to. We do know that the problem happens at 30% left on the battery. We also know from their work and studies that, and it's estimated, your slowdown might happen anywhere from 70% to 30%.
Why would you slow the phone down if there's 70% left on the battery if you know the problem is only going to happen when there's 30% left? Why would you degrade the user experience right at 70%?