since we all like things to work faster, i've been working with the wonderful archives from http://www.hvsc.c64.org/
(since a 32k file archive is the best i can find to push in_zip to it's limits which i'm sure you'll find that it will do).
the result as of last night was a minimum of a 2 second time saving (will vary on different machines) when viewing the alt+3 dialog on the archive which was consistant over the rar,zip,ace and 7z versions of it i made/tested. 2 seconds probably doesn't sound like much but when it was taking 6 seconds to fill the list to begin with it's a nice increase (dependant on system load at the time i could nudge a 2.5 second time saving
i've still some more work to do to improve the handling of the extraction and title reading from such a large archive (it causes the playback of the sid files to skip when it's reading in all of the titles (with a few seconds delay at the time which isn't ideal at all).
the end result though is that for normal archives (when the work is done) there should be an improvement in overall speed of the plugin (it won't be as much as for the large archive's but there'll be some sort of increase verging on placebo levels maybe, heh