There's nothing stopping a mail client from maintaining arbitrary indexes, caches, etc. in addition to the maildir.
You know why mutt can be for me? I have a second program (isync/mbsync) sync the maildirs, a third to index it, and mutt is just for reading and sorting.
When you have one monolith doing all 3 at its will all the time, you get calls from users that Thunderbird and Mail.app are janky and the computer is hosed. It is interesting as FTS came to Thunderbird and they ran the indexer all the time, until they intelligently designed to have better controls around its execution as it destroyed general usability. Mail.app is better, but I see executive running it for days and it becomes a misbehaving pile of mess.
I am asking in a respectful tone and not to make light of your position, but what do you use? I want to get a better look into your view and I think this might help me understand you better, if you are willing to indulge me.
UPDATE: And when I say several things at once, ironically this is not by design! Email has changed a lot since the sendmail days (and that is cutting edge with respect to many veterans here) and thus there is well-known and accepted design I can think of beyond layers of addition on clients to meet definitions and changes in protocol, routinely made without any discussion of client operation beyond standards for file structures like mbox and Maildir, even they are formal standards, I would have to check,
Well OK, so the most important for me is search, and the second most important is being able to quickly apply labels based on filters on a bulk-scale. I have found mutt to not really be performant and the workflow that I achieved was rather brittle (complicated by other factors like having to deal with HTML email). I'm currently using Gmail, but I am experimenting with outlook and I quite like the performance. I don't have a ton of email, maybe around 3-4GB, 25K emails. Bulk operations like 'select all' - do X on several thousand emails takes seconds (which is still slow IMHO FWIW). I haven't seen such performance with other clients. Granted, I haven't broken down every single client and benchmarked different workloads, so I'm only speaking in generalities.
And don't worry about offending me. I was just trying to stir up some conversation. I don't take offense if someone disagrees with me on the internet.
As I suggested, I slip everything up into different applications. I use mutt-kz and not much with isync to sync maildir, view it mutt, and do label/filtering like Gmail locally with notmuch on a Xapian FTS subsystem backend. It's fun.