The apmail listing jobs (mods, subs) are generally quite expensive to run. However the output does not change very frequently.
So a possible approach might be to run a cheap(er) check to look for changes to the source files and only run the extraction when there is a change. This should allow list-subs and list-mods to be run hourly rather than 6-hourly. I'm happy to look at how to implement this if people think it's worth pursuing?
