I hope this is helpful: First, following Apple's advice I restored my entire iPhone which is a big pain, and forced me to pay for more iCloud storage, which feels a lot like a shake-down. The restore helped maybe a little bit, hard to say, but Mail was still not really functional. Deleted and re-added accounts also, and that did not help. Apple support was not particularly helpful, since their operating assumption is that everyone is completely clueless and hasn't done simple things like rebooting their phone.
So I tracked my problem (blank screens, crashing, laggy response, mail disappearing...) down myself to my work-related account, by deleting each account and seeing how they behaved. gmail itself was OK, my work-related account was not good by itself. I talked to my work sysadmin who immediately figured out what was happening, which was that about 25 years ago (!) he set things up to deal with everyone's different mail files, and that decision meant that everyone's /home directories were being treated as mail directories. Well, people (like me) dump a lot of stuff in those /home directories, and what I think has happened is that Apple has started to cache EVERYTHING in Mail directories to the phone. If you have more junk there than the phone has RAM, it either swaps it to disk (if you have enough) or it just fills up the RAM and crashes. I tested this by getting rid of lots of old junk in my /home directory on my server, and sure enough Mail is much better (but still not what I would consider "normal" or satisfactory, but at least functional.
What does this mean for you? Well, it might be that some people have a huge number of old mail messages either in their inbox, or in archived mail, or drafts, or spam, or whatever. And it may be that now Apple is trying to cache all of that stuff on the phone and using up all the RAM or trying to swap (which is slow). So some of you might try deleting all your spam (for example) and seeing if that helps, or even (gasp!) deleting old mail, or backing it up somewhere, etc. If you click on your account after opening mail (if you have more than one, it will be in the shaded lines below the line with the "envelope" icon) you will see what Mail thinks is in your directory on your host. If there are a lot of things that don't look like mail (other files or whatever) then that probably does mean you have stuff in there that is slowing things down, and you may have to figure out how to remove it on the host itself.
All of this has come up with iOS 13.1.3/13.2 because Apple has done something in how it handles IMAP. While there may be good reasons for it (I would guess they thought it was faster; or maybe they have AppleMail that they want to force people to buy...) there is no excuse for the Mail app to crash or behave so poorly without at least providing an error message like "Mail can't load host directory" or whatever.
Good luck.
