I have now updated my iphone and ipad to ios 18.3.1 but still my email doesn't work. I've removed all my email accounts except iCloud. I've sent a test email from my Mac to my iCloud account (just says "hello, how are you" -- no attachments) and it takes about 15-20 minutes before it appears on my iphone and ipad -- the little wheel just goes round and round till the email finally appears --I'm just so fed up with this. My iphone is a new one and it's now rubbish. Even iCloud mail is so slow, never mind my third party accounts (Yahoo, Microsoft).
Things I've tried: 1) removing accounts and adding them again, (2) restarting the iphone & ipad, (3) turning VPN off, (4) changing iphone mail to List View as BopCat suggested. None of these makes any difference.
Question: someone suggested that Categorising is going on in the background even when you've selected List View -- does this slow things down? Even so, the Categories/List View is only available on my iphone, not the ipad. The ipad doesn't have the Categories/List View option at all, and it's still really slow.