I have a very similar problem and try to analyse the Issue with Wireshark.
I reporduce it this way - go to an iPhone and try to get the body of an old email - for some iPhones (but not all at the company) recieving the body needs more than a minute - until then, only the "pls wait sign" is on the screen of the iphone. Then suddenly the email content becomes visible.
I downgraded from TLS1.3 to TLS1.2 to be able to decrypt traffic with the priv key from letsencrypt in wireshark. I don't know much about IMAP but it seems that the UID FETCH command is simply executed 1-2 mins AFTER the user tried to receive the Email-Body.
Here a Screenshot with the Time passing in the red rectangle: https://drive.google.com/file/d/1jTajdI6XDTAXNZVUdE1VYLeXi8S6jreA/view?usp=s...
First insight MAYBE that the bug is already fixed - I have two phones which currently doesn't seem to have this problems which have one version number up compared to the ones which have this bug. Their version numbers according to the IMAP Hello (or whatever you call that if you are an IMAP geek):
iPhone with problem: FK2 ID ("name" "com.apple.email.maild" "version" "3826.400.131.2.14" "os" "iOS" "os-version" "18.3.1 (22D72)" "vendor" "Apple Inc")
iPhones without the problem: AK2 ID ("name" "com.apple.email.maild" "version" "3826.400.131.2.15" "os" "iOS" "os-version" "18.3.2 (22D82)" "vendor" "Apple Inc") I2 ID ("name" "com.apple.email.maild" "version" "3826.400.131.2.15" "os" "iOS" "os-version" "18.3.2 (22D82)" "vendor" "Apple Inc")
I will tell the person he should try to update the iOs version and will update you here in case this fixes or doesn't fix it. Thank you!