something is wrong and I bet that the page cache is not working - maybe the supercache configuration is not right or something with the server is not right (perhaps, Nginx configuration). The source of your pages say "Dynamic page generated" - meaning this could be a non static page.
I can't do it from here, but you need to test:
- Is supercache creating a static file
- Is the static file really served to the visitor?
- Can you put a screenshot of your SC configuration?
Also, you can minify and combine your CSS and JS (use Autoptimize for it), but that's not the main problem