Sorry I can't help you from here, I didn't use it and didn't read about it.
If this can help
PageSpeed configuration
Or look at on some tutorials from web hosting blogs.
Maybe someone will join this discussion who know little more about it.
How You can check if mod_pagespeed works?
You can check Gtmetrix and check your caching and page speed.
First disable any other caching and check plain Website.
write or make picture of results or leave it open in one tab.
Then install and activate mod_pagespeed and check your website again (on other tab) then compare results.
See if there some suggestion what do you have to do and try to search answer for that.
After you done everything enable all other caching and check your website again and see final result.