Sadly, worrying about a phone slowing down over time is something that happens with both phones. I don’t want it, but battery technology is what it is. And being Open Source hasn’t helped bring it forward with the amount of “Android doesn’t slow down your phone like this” comments I’ve read.
Sadly, apple has a reputation of putting smaller battery than its competitors in the past, hence shorter battery life span. The plus model phones are some of the largest in the market, yet apple only includes sub-3000 mah battery in them.
If they knew battery isn't going to hold the same charge / voltage in a year or so, they should upgrade the battery instead of slowing down the device through software.