You want to provide a directory listing but want to reduce the performance hit of doing so.
Use the TrackModified argument to IndexOptions to allow browsers to cache the results of an auto-generated directory index:
When sending a directory listing to a client, Apache has to open that directory, obtain a directory listing, and determine various attributes of the files contained therein. This is very time consuming, and it would be nice to avoid this when possible.
By default, the Last Modified time sent with a directory listing is the time that the content is being served. Thus, when a client, or proxy server, makes a HEAD or conditional GET request to determine if it can use the copy that it has in cache, it will always decide to get a fresh copy of the content. The TrackModified option to IndexOptions cause mod_autoindex to send a Last Modified time corresponding to the file in the directory that was most recently modified. This enables browsers and proxy servers to cache this content, rather than retrieving it from the server each time, and also ensures that the listing that they have cached is in fact the latest version.
Note that clients that don't implement any kind of caching will not benefit from this directive. In particular, testing with ab will show no improvement from turning on this setting, as ab does not do any kind of content caching.