ThisSame here. I’ve got 1 Exchange account along with my iCloud account, 3 Gmail accounts, a Yahoo account, an Outlook.com account, and an IMAP account. I’ve only seen worse performance over time with every release- official or beta releases... Its my iCloud account I find I have the most problems with. That, along with the “All Inboxes” folder. The (still open) Radar I filed with Apple back in mid-August is still open with a note at the top saying a fix might be available in iOS 13. How many releases of iOS 13 have been released now? Not including the initial Dev betas before the official release of iOS 13 (not iOS 13.1)?
But I have to say, on b3 if I open the mail app and pull to refresh using only the Exchange account Inbox page, it resolves E.v.e.r.y. time. Without. Fail.
If I pull to refresh using only the iCloud Inbox page, it may refresh in a second, or it may take up to 30 seconds to refresh. So far in the last 24+ hours on b3, it has always eventually refreshed, so maybe a little better than b2 - for me. Just sharing my experience.