A/C vent barely blowing out air

The A/C works and blows great in town, but when I take it on the highway, after about fifteen minutes there is barely anything blowing out of the vents even on high. It does not matter what setting it just barely blows anything out. What does barely come out of the vents is cold but it is not enough especially on a hot day. Can you tell me what is wrong and what I can do to fix it? Thank you.