Short version: My 6S with a "healthy" battery is severely throttled. It normally runs now at 911Mhz and often less. It is practically unusable, and it happened suddenly, recently. Something about Apple's story doesn't add up still.

Longer version: My 6S battery got replaced back when the sudden shutdown at 20% thing first started happening. (I actually got them to do it out of warranty for free.) One app says it is at 87% health 1500mAh vs 1715mAh original capacity. I've read that Apple won't even replace the battery for you until it drops below 80%, but my phone is unusable right now. Basic animations don't work right. (I might get two frames of an animation.) Siri is unusable, as another commenter described. Another app reports the speed the phone's processor is running at. The majority of the time, mine is throttled to 911Mhz. Sometimes it's a bit more, if my battery is at 99-100%. Other times it's even lower. This is a 1.8Ghz processor, according to what I've read online. Turning low power mode off and on doesn't change anything.

Something just doesn't add up about this.