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ezmlm-idx adds a number of functions to ezmlm. It indexes the archive, and adds an index entry for each message, it can remove MIME parts, it can add a subject prefix and message trailer, decode RFC2047-encoded subjects, etc. Although designed to impact minimally on performance, these options when used take time. Even when they are not used, time is spent looking for e.g. the prefix. However, the performance penalty is small, as the absolutely dominating cost of a mailing list is the work qmail does to deliver the messages to subscribers.
In bench marking, we have not found a significant difference in performance between ezmlm-0.53 and ezmlm-0.53+ezmlm-idx-0.32 when ezmlm-idx features are not used. Thus, a non-indexed list with ezmlm-idx-0.32 performs the same as the corresponding ezmlm-0.53 list. Adding an index adds the overhead of another safe write (the index file). Use of other features adds very marginally to execution time. For virtually all lists, the ezmlm execution time is negligible compared to the resources needed by qmail to disseminate the message to the subscribers.