I have this identical problem on my iPad mini, and oddly, I am not even on ios15!
All was working perfectly a week ago, I have been away without the device so haven’t changed anything, and suddenly the email is unusable.
Apples tech guy wanted me to ‘upgrade’ to 15 or remove and reinstall my email accounts, but as there are 2 of them and they are both still working perfectly on other devices, the problem is clearly with apple’s software.
He even said if I experienced problems after upgrading, they would refer to tech team to sort! Ridiculous that customers should be expected to do their debugging for them ....
The iPad is less than a year old - this is clearly unacceptable