Incoming traffic slow

Not sure whether I should call this inbound or outbound traffic. We use the Balance 50 in the office with 2 WAN connections, one is on a 100Mb fiber network, the other is on a 40Mb DSL. We have an internal webserver we use in the office for development and things work super fast locally as well as browsing the internet / FTP and so on. Speedtest gives us a result of around 130Mb up and down.

After setting up port forwarding of port 80 to the webserver so that I, and customers can get to the internal server from out of the office things run horribly slow. It could take over a minute to pull up a page that loads in 2 seconds in the office. Is there anything I should be aware of for this intended use? Is there more to optimizing the connection that just setting up the forwarding?

Just the port forwarding rules is all that is needed. Does this happen when you come in from both connections or just a specific one?

I never thought about changing to the slower connection, I had it on the 100Mb fiber. I will try it tonight with the DSL.