See 7a70ec71-3686-4468-8647-4d6da0240727. This seems to be a recent and persistent issue. I've rebuilt the global index and several mailboxes. Activity Manager shows a real drop in the performance of indexing a mailbox (it gets slower and slower), eventually resulting in a crash.

Side note: the message "Indexing {X} of {Y} messages in {folder_name}..." would be infinitely more helpful if the message also mentioned which account that folder was in.