[SuperNinKenDo]

The logic is probably as discussed above. Opening files is relatively expensive on Windows, so intuitively, maildir should be worse in terms of performance. I believe there's also some filesystem reasons to prefer avoiding lots of small files but that's beyond my pay-grade.

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.