I understand MTU to be the speed of the water flowing through the pipe. No matter the size of the pipe (the potential speed), it can flow at full blast or a trickle. If its a trickle, its an MTU issue. I see this at hotels all the time, especially when wireless. Often certain services like xBox Live demand a minimum MTU before allowing you to connect. My kid found this out the hard way at a Hyatt Place recently.