Both my and my wife's one-and-a-half year old iPhone SEs were experiencing the Apple iOS slowdown problem, and everything I read said to replace the batteries. This works by the way.
I found a YouTube video online that really aided in the repair. I found the most difficult thing was managing the tiny screws and clipping the electrical connectors back in place. The other very difficult item was both sides of my double sided tape broke. I ended up having to pry the battery up part way to get ahold of the broken ends of the tape.
Make sure you check very carefully which connectors need disconnecting. I unplugged one that didn't need unplugging and it was difficult to align back up.
One thing I wish is that factory double sided tape is sent out with replacement batteries. I didn't need a second kit including tools for my wife's identical SE. So I just ordered the battery and that is all I got.