Howcome mail deletion time varies?

[Chuck Robey]

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I use, for my imap-based mail, a combination of postfix, dovecot,
thunderbird, enigmail (for gnupg), and openssl for browser security.  When
I delete mail messages, the majority of them delete (what seems to me to
be) instantaneously, but a small minority of mails takes quite a bit
longer, about maybe 20 seconds.  Any idea what might be occurring on those
mails, to trigger this really long delete time?
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