So I've done more digging. For full disclosure, the server is protected with a SSL certificate. Turns out if I point the web viewer at:
the web viewer renders without issue, both in FMP and WebDirect.
But this is kind of going outside the local network to the web to download the html file yeah? I was anticipating it would load on the local network when being displayed in WD or FMP - which is where the speed increase would come into effect I think.
Scratch this, I think I've figured out what's needed!