I don't think the vent on top of the tank being plugged or capped would slow your fueling since there is a vent hose next to the fill hose. Hard to imagine the vent hose getting clogged since it is at least 3/8" id. Easy to push a wire though it to check though. It could be that the roll over protection ball in your fill hose is stuck part way up. It's easy to get at by removing the fill hose, first remove the screws by the cap then the hose clamps underneath. Some folks have removed the ball entirely for very fast fill ups.