5-6 mins/gallon is extremely slow, assuming you are not exagerating. I don't think that will be from a vent problem. It could be the fill hose is kinked. It could also be something has become lodged in the fill tube, like the bottle cap from the top of a Techron bottle, for example. See if you can get a siphon hose to go all the way down the throat.

The plastic tube part that penetrates the tank, and connects to the fill hose, is only about 1" diameter.