This is generally a good "last resort" solution and becky_s22 describes the process well. But to save everyone the trouble, we've tried this on two devices running 11.0.1 and it has not helped. Unlike some of the other posters, we use all IMAP accounts and no Exchange services whatsoever. It appears to be an as-yet unresolved bug in iOS 11, and actually, I see similar behavior in multiple apps - not just Mail (i.e., many apps are slow to load or establish network connection).
I can literally login to my Mac, fetch mail, read my email, maybe send a short message, and logout before iOS 11.0.1 has fetched new mail.
If I watch on my WiFi and see what Mail is doing on iOS while it is spinning waiting to receive Mail, the answer is - not a thing (on the network at least). I see that with Photos, iTunes, and many Apple apps.
I do not see a stall in third-party apps. The couple games I tried hits those ad servers lightning fast. So I suspect it's a big in some library or code common to all the Apple apps.