I just updated my iPhone 6s from iOS 10.x to 11.0.3 yesterday. Now the Mail app is VERY SLOW to display/download the full message. I see a preview (2 lines) of the message in a timely fashion, but after I click on the message to see the full message, just "LOADING" displays with the silly spinning wheel in the bottom 2/3 of screen. It can take several minutes before it finally displays the message - even for ONE LINE "test" messages. I am using a Microsoft Exchange email account. My Mail worked just fine on my iPhone under 10.x, now it's limping along. My iPad (still iOS 10.x) gets mail just fine, connected to same wifi network, sitting inches away from iPhone as I do side by side tests. I called Apple Care and even spoke with a senior tech - she was totally clueless! I asked her what search terms she was using to check her (Apple) database to see if this is a known bug? She didn't respond for a long period, then said: well, I just searched for "Mail"... I'm sorry, but a child would have known how to search more effectively/intelligently! I suggested she search for keywords like: MAIL, EXCHANGE, LOADING & SLOW. She was offended that I was telling her how to do her job! I told her I've been doing IT support for 30+ years (albeit Windows). Then I asked her if Apple monitors their own community forums and gathers potential bug reports from all those good customers that take the time to post messages about problems? She said NO!!! At this point I have ZERO respect for Apple - as far as support goes. They have the brightest minds doing the hardware innovation, but then hire clueless, unambitious people for support. Bottom line - use Google searches to solve your Apple problems - it's a much more pleasant experience and you'll solve your problems faster and easier...
As for my iPhone and Mail/Exchange woes - I deleted my Exchange account and recreated it. I have several hundred sub folders under my INBOX, so it will be some time before I can test.
Oh, and just to emphasize how lackluster Apple support is: I tried filling out a bug report on their Feedback page. On the feedback form there is a drop down field to indicate what iOS version you are reporting. Well, it only lists up to 11.0.1! I'm using 11.0.3. Duh - who maintains this important part of their website? Somebody is asleep at the wheel... 😟