I ran the site against pagespeedtest.com, and it failed on image compression. Is there a plugin that will optimize images? Or is this something that should be done on the backend?
Please provide a link to your Coppermine site. No one can see exactly what the issue is.
(Board rules too)
Run the same test on some other image-centric sites. You'll probably see similar results. Image sites are supposed to have good images that are not overly compressed.