It's too bad that our editor had to see this performance bug---it's just
the kind of thing that might make him grumpy.
Upgrading Xapian should make notmuch quite snappy when making tag
manipulations such as removing the "unread" tag while reading messages.
The performance bug is fixed in the upstream Xapian versions (1.1) as
well as backported to Xapian 1.0.18 (which appears in Debian testing now,
But all in all, a very reasonable review. Thanks, corbet!