Why, exactly? I have switched to maildir as soon as it was available as experimental feature, and performance gains when compared to mbox were enormous, especially during bulk operations. Switching folders takes <0.1s, with ~100k messages per folder, on Windows 7 64-bit.
The reason maildir is faster despite this is the antivirus factor.
The fastest solution is adding an exception so that Defender doesn't scan your Thunderbird email, however that has the trade off that your antivirus isn't able to scan your email.
And while switching folders, which is the major part of UX anyway, is fast, because TB only scans a handful of messages in the view[0], what about other operations which would need to scan the entire mailbox, like searching for something?
[0] why even it does that? beats me, but clearly it does, otherwise you wouldn't see the speed improvement