I have a customer who I've just upgraded their iPhone XS to an 11 Pro, using the standard upgrade option of having the new phone next to the old one, and doing a direct phone to phone transfer.
Now, the old phone (just on wifi) updates email instantly, whereas the new one with WiFi and 4g can take up to 45 minutes to update email! It's very, frustrating and the customer is livid as he got this phone to be better.
They're using an Office 365 email account, and that gave problems originally as well, as the Exchange console on 365 showed the device as 'disallowed' and I had to delete the account and add it manually, after deleting the phone from the Exchange device console.
Something not right here, and I'm taking the flack for it.