IMO it is recommended to ignore 50% of the google speedtest recommendations, this is one of them
Resources with a “?” in the URL are not cached by some proxy caching
servers. Remove the query string and encode the parameters into the
URL for the following resources…
Maybe it was true in 2005 but now I wish google would have specified exactly which are they. In any case the chances of any page of any small or medium site to be cached and reused from the cache is minimal. The effort you might need to put in rearranging the urls of your static resources is not worth the zero speed improvement your site will gain.