We want to serve ads on our site but the adserver we are in talks with has issues with delivering their advertising fast enough for us.
The issue as I see it is that we
To decouple the main page loading from the ad loading, you can put the ad in its own page in an iframe or, similarly, download the script file with AJAX and execute it whenever it comes down. If the former is not adequate, because of referring URI or whatever, the latter gives you some flexibility: you could use string replacement to rewrite "document.write" to something else, or perhaps temporarily replace it like "document.write = custom_function;".