arduino webserver serves single web page to WAN/Internet very very slowly
I haven't used the ethernet library for a while, but I remember that it served up tiny packets - basically each write call went in its own TCP packet. It sounds like you may have latency issues across the WAN. 

You could try using your Apache instance to proxy the Arduino pages. Details are at http://httpd.apache.org/docs/current/mod/mod_proxy.html I have this running but all my pages are small so I'm not sure how it would behave on larger pages.