My iPhone 6s’ battery has been losing efficacy over time, and its processor has been performance-throttled for the past two months. So, I’m sending it to Apple for a battery replacement. I opted for the mail-in procedure because I’m lazy.
I’ll be without my phone for “five to nine business days,” which sounds like two weeks to me. I haven’t been without a mobile phone since 1995! (My first phone was an analog Motorola microTAC.)
Seek Nuance 
Sent mine in 2 mornings ago and I just got a notification last night that it’s repaired and they’re sending it back. Hopefully it all goes smoothly. So far, so good.
I meant to post my experience but didn’t get around to it.
I got my phone back in like four days. The phone is like new!
I had to reset it to factory settings and then restore it from my last iCloud backup because it was in an odd state. That was easy (just had to wait an hour) and since then it’s been perfecto.
I was planning to upgrade to the next new model (our upgrade cycle is once every three models…) but I may skip it because this is now completely satisfactory. I’d forgotten what it was like to not need to recharge my battery mid-day!
I had basically exactly the same experience. Dropped it off at UPS on April 2 and got it back on April 6. It did take an hour or more to restore from iCloud backup, but I have a lot of apps/photos/videos, etc.
New battery is readily apparent — very strong.
I love this phone and hope to use it for 2 more years after this upgrade.