I'm also using Mail.app on my macs and while I never had those problems I do see people complaining about those things a lot and I'm curious what would cause that. Apple's radar/bugreport/feedback stuff is hidden so that's somewhat sad (but understandable) but maybe it turns out most people have the same problem due to similar context (account setup, data in use or something...).
The most heavily loaded Mail.app I seem to have is one with two MS Exchange accounts, iCloud, a couple of IMAP accounts and a single POP account. I generally archive everything older than ~10 months out of my inbox leaving a combined 43k messages in that virtual inbox 'group'. Maybe people with larger message stores trigger some programming fault?
Possible. For me, it's a bunch of small things. I have three mailboxes. For one, search is almost always slow and incorrect. To the point where I usually have to load Gmail's web UI to be confident that I haven't missed anything.
It's strange because I like a lot of what Mail.app does—it's simple, no bells and whistles—but the search experience, etc. just ruins it entirely.
Some of the IMAP accounts I have are also Gmail and GSuite accounts, search seems to work fine (both from the global search thing and per-account). Or maybe I'm actually missing results but I don't know about them because I'm not seeing them (but somehow still finding what I need).
The most heavily loaded Mail.app I seem to have is one with two MS Exchange accounts, iCloud, a couple of IMAP accounts and a single POP account. I generally archive everything older than ~10 months out of my inbox leaving a combined 43k messages in that virtual inbox 'group'. Maybe people with larger message stores trigger some programming fault?