why does mutt take so long to load my python folder?

[Cameron Simpson]

I'm using my python folder (a Maildir) because it is large enough to do 
a little live debugging on. Summary header:

     cs at cskk.id.au reverse-threads/last-date 0% 1152M Msgs:140928 New:90812 Del:0 Flag:608 =python

I'm using a header cache, and my sort is reverse-threads/last-date.

Command line:

     mutt -f ~/mail/python -e fcc-hook . ~/mail/python; save-hook .  ~/mail/OLD/2022/python

(That's from ps; the `-e` argument is a single string of course.)

The stuff before the "Sorting mailbox..." phase seems ... fast enough.

During the "Sorting mailbox..." phase I see mutt reading message files 
I'd not expect it to need to touch, _and_ making some temporary files.  
Example `lsof` output:

     mutt    1162 cameron    3r   REG    1,5     8370           143651075 /Users/cameron/mail/python/cur/1626370570.#7159M542304P658:2,S
     mutt    1162 cameron    4u   REG    1,5        0           599663041 /Users/cameron/var/mutt/mutt-fleet2-lan-501-1162-13395134689776834598

That `cur` message file dates from 2021-07-15 17:36:10 GMT (long ago) 
and the temporary file is indeed temporary - it vanishes. I speculate 
that mutt's... unpacking the source message in some way?

I had expected that the sort would happen in memory using the header 
cache to obtain values. ISTR that the header cache only caches 
particular headers, but I can't see which in the docs and would have 
imagined it at least caches the headers used to sort messages.

Thoughts?

I append the output of `mutt -v` below.

Cheers,
Cameron Simpson <cs at cskk.id.au>

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ncurses: ncurses 5.7.20081102 (compiled with 5.7)
libiconv: 1.11
hcache backend: tokyocabinet 1.4.48

Compiler:
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Target: x86_64-apple-darwin19.6.0
Thread model: posix
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Compilation CFLAGS: -g -O2

Compile options:
-DOMAIN
+DEBUG
-HOMESPOOL  +USE_SETGID  +USE_DOTLOCK  +DL_STANDALONE  +USE_FCNTL  -USE_FLOCK   
+USE_POP  +USE_IMAP  +USE_SMTP  
+USE_SSL_OPENSSL  -USE_SSL_GNUTLS  +USE_SASL  -USE_GSASL  +USE_GSS  +HAVE_GETADDRINFO  
+HAVE_REGCOMP  -USE_GNU_REGEX  
+HAVE_COLOR  +HAVE_START_COLOR  +HAVE_TYPEAHEAD  +HAVE_BKGDSET  
+HAVE_CURS_SET  +HAVE_META  +HAVE_RESIZETERM  +HAVE_FUTIMENS  
+CRYPT_BACKEND_CLASSIC_PGP  +CRYPT_BACKEND_CLASSIC_SMIME  +CRYPT_BACKEND_GPGME  
-EXACT_ADDRESS  -SUN_ATTACHMENT  
+ENABLE_NLS  -LOCALES_HACK  +HAVE_WC_FUNCS  +HAVE_LANGINFO_CODESET  +HAVE_LANGINFO_YESEXPR  
+HAVE_ICONV  -ICONV_NONTRANS  -HAVE_LIBIDN  -HAVE_LIBIDN2  +HAVE_GETSID  +USE_HCACHE  
+USE_SIDEBAR  -USE_COMPRESSED  -USE_INOTIFY  
-ISPELL
SENDMAIL="/usr/sbin/sendmail"
MAILPATH="/var/mail"
PKGDATADIR="/usr/local/Cellar/mutt/2.2.7/share/mutt"
SYSCONFDIR="/usr/local/Cellar/mutt/2.2.7/etc"
EXECSHELL="/bin/sh"
-MIXMASTER

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