Well if you actually run the site on port 8080 it will work but the problem is the site is really running on port 80 and you are redirecting requests from port 8080 on your firewall to port 80 on your web server. But since the web server is really running on port 80 and does not know anything about the redirection, it does not build internal urls with port 8080 in the url and therefore these urls fail because they don't specify port 8080. This includes the urls for the css. Since the urls are not accessible the page is slow because it is waiting for the requests to timeout on these bad urls.
If you change the web site to run on port 8080 and just map 8080 on your firewall to 8080 on your web server then it will work.
Hope it helps,