I have a site included lots of external files, most of them are gif format. I have no control on the external files, but have to use them(with permission). When I check the site using Google Pagespeed, I got very low score(31) even though the page load is fast.

One of the high priority suggestion is to leverage browser caching by setting an expiration date. However, all the files are on external links. I have already set the expiration date for local files.