The phone was slow and the battery was not lasting very long. I used an external battery case for a few weeks to keep the phone alive and was planning to upgrade to an iPhone X. Then the scandal broke out and I read a news article about the iFixit battery kits for my phone and thought I'd give it a go.
It went very well. It was very challenging but only because the screws are so very tiny. The hardest part was getting the thumbprint scanner clip to snap into place. After about 30 tries I got lucky. Hopefully I won't need to open this unit again. I have checked out the teardowns in the past out of curiosity but after this initial repair experience, I would trust an iFixit kit and instructions to fix just about anything.
When I reassembled the phone the first time I neglected to pop the TouchID clip into place properly and Touch ID could not be activated. Luckily, the instructions indicated what happened there and I was able to just pop the battery connector and replace it, then properly seat the clip. The phone has 100% functionality and seems faster but I have not taken any quantitative measurements. Go slowly. Be methodical. Take breaks. It's great seeing the charge lasting like it did when the phone was new and not having to use that clunky battery case.