I'm not sure what you mean by "distinct GAS server", but based on what you say later, are you meaning that GAS and Apache2 are on separate servers.
I'd expect a setup where the Web Server and the Application Server are on the same server to outperform one where Web Server and Application Server are on different servers (all other things being roughly the same). The reason for this is that files have to make two hops instead of one to get from the App Server to the browser. This is where the WEB_APPLICATION_PICTURE_COMPONENT
https://4js.com/online_documentation/fjs-gas-manual-html/User/ASAppPictureRef.html#PATH comes in handy to position files on the Web Server instead of the Application Server.
The other thing with slow rendering web pages is to use the various browser debug tools available to find out what files are being loaded and how long it takes to load them. For Chrome press F12 and then click Network, and then refresh your web page.
Also if the delay is a multiple of 5 seconds, this normally indicates some sort of DNS error.
Reuben