nmh + notmuch: expected to work?

[Eric Gillespie]

I'm a long-time nmh user, and I heard notmuch was supposed to
handle mh folders, so I gave it a shot.

It's crazy slow.  The first 'notmuch new' took 4 hours, but maybe
that's expected.  However, every 'notmuch new' takes 4 - 10
hours.  The only time it completes faster is if I immediately
re-run it without incorporating new mail.  In that case, it
completes in 0.8s - 38s, which is quite reasonable for my mail.

I have 18.5 GB, 1,058,370 messages in 315 directories.

From the messages it prints, apparently any directory that
changes (receives a new message) has to have all its files
indexed from scratch.  What?!

# re-index after some new mail (< 1000; not 704083!)
Processed 704083 total files in 11h 7m 48s (17 files/sec.).
Added 701791 new messages to the database. Removed 701332 messages. Detected 2136 file renames.

# re-index again, no new mail
Processed 60 total files in 40s (1 files/sec.).
No new mail.

# incorporate 1 new message
Processed 9181 total files in 7m 54s (19 files/sec.).
Added 9071 new messages to the database. Removed 9071 messages. Detected 12 file renames.

# incorporate 1 new message
Processed 111 total files in almost no time.
Added 1 new message to the database. Detected 12 file renames.

# incorporate 1 new message
Processed 757 total files in 44s (17 files/sec.).
Added 645 new messages to the database. Removed 645 messages. Detected 14 file renames.

There haven't been any renames at all, so I don't know what 2136,
12, or 14 renames notmuch is talking about.  I wasn't reading or
replying to any mail at all, only adding new mail and running
notmuch new.

I already thought mairix was slow, but in comparison, it's
blazing fast.  I think notmuch indexes much more so the
comparison is not entirely fair, but at least it's usable.

mairix takes 1h15m to process the same mail from scratch.
Subsequent updates take from 19-22s.

Also the mairix db is much smaller, though notmuch's is about the
size the documentation says to expect:

0 ~% du -ms .mairix mh/.notmuch
447     .mairix
15712   mh/.notmuch

Is this expected?  Is this attributable to some bug in how
notmuch handles mh folders, or does this happen with maildirs?

Thanks.