I had originally had a G'Bar appointment for them to look at it in Jan; they called me and said they were out of stock so call AppleCare and set up the repair, and indeed the AppleCare person on the phone did run remote diagnostics (which involved a bit of button-pushing on my end). Then she proceeded to set up the battery order, so I guess it looked bad
It's not very painful but it IS tedious, so block out some time and preferably call from a different phone (Google Voice on your laptop, maybe?) I suspect claiming that your primary problem as grotesquely reduced battery life (rather than performance) might smooth the path, not sure tho'
It actually was mine, too -- battery life deterioration. I had thought about getting a new battery (for this 6) last fall, depending on what the new models looked like. THEN the performance slowdown issue erupted (and I saved $50 on the battery )
Glad you got that resolved. I had a third party replace the battery in my iPhone 5 earlier this year. Good thing I did, too - the old one was puffed enough to put pressure on the back cover.