i'll tell u whut...
i've got an iphone6 which had a failing battery about a year ago. took it to apple and their stupid diag said the battery was fine. 3 months later i went back with even worse battery life and they told me it was still good, but because it had 700+ cycles they would let me pay $80 to replace it. that was march 2017.
everything was fine until i installed iOS11... and then the massive slowdown began. i just assumed it was the software, but geekbench 4 and cpu dasherx have shown me that i'm getting massively throttled. if my battery is below ~80% my CPU is stuck at 600Mhz. meanwhile, my kids iphone6 (both still running ios10, both purchased the same day as mine, and both on original battery) are totally fine speed and battery life wise. so how is apple not causing this slowdown???
OK, so i've got a 9-month old battery and i'm getting the snot throttled out of me. i know, KNOW, that if i go into the store they will tell me to pound sand and that there's nothing wrong. but my iphone6 is completely unusable at this point.
so to me, the $29 iFixit battery makes sense, at least for the 6 where i don't have to worry about waterproofing. and i can do it myself instead of wasting hours pleading with some kid to make me whole. that's what's frustrating. they brick my phone, then i have to jump through all kinds of hoops to get it to work again. F that.
i've been an apple user since 1978 when i got my first apple ][. i've probably spent over $200,000 on apple products over the last 30 years. i'll never use an android phone, but from this point on i'm going to buy used iphones and take my chances. i'm never going to give apple another dollar for a new iphone.