Did you have it replaced at an Apple Store or by a third party? I'm deciding between the two – my gut is to go with the Apple Store, but the apple website makes it sound like it could take up to 5 days, which is too long.

I'm based in London, so dropped it off with https://www.ismash.com/ in St Pancras. Was done in half an hour and cost £40. Can't vouch for the quality of the battery yet, but they are a pretty big chain now so I'm assuming it won't be awful.

However I've had batteries replaced (different phones) at the Apple Store before and it took about an hour.

What app or site are you using to test that?

Geekbench https://itunes.apple.com/ca/app/geekbench-4/id1130770356

And what battery did you (GP) replace the original with?

It looks like Geekbench 4.

It's Geekbench 4