Immediately I noticed my phone was basically unusable, incredibly slow, everything was crashing or locking up completely, I couldn’t believe it. I thought iOS 11 was the culprit, and even did a complete reinstall without restoring any backups to no avail. Finally, I noticed a slight dark spot on the LCD - the brand new battery I had bought was already bulging and in bad shape.
I sprung for a better quality battery from ifixit and immediately the phone was running like brand new again. I thought the cheap battery had been supplying an unstable current, but perhaps there is indeed an issue with the software itself.
One key thing I realized - ANY battery work done outside of Apple’s official repair means they will never work on your phone again. So if you care about having Apple do any kind of work on your phone (even if totally unrelated to battery), you will have to get it replaced by them.