+The performance tests in the notmuch source didn't show any real
+issue, even on a non-encrypted ext4 fs.
+
+After investigation and running different tests, a switch to a
+LUKS-btrfs based filesystem to replace the ext4-ecryptfs resolved the
+issue. It now takes some seconds to index hundreds of messages.
+
+## Xapian database format: chert vs. flint vs. glass
+
+Depending on the Xapian version that was used during the initial notmuch
+database creation, you might use another database format than the most recent
+one, which is often faster.
+
+Check whether you are on chert, flint or glass: