vent pipe for fresh water

If we are talking about a gravity fill port, the problem is often with the fill hose itself, not the vent. The fill hose needs a good slope to it, and if there is a low spot, or if it is simply too long, filling will be slow. There may not be much you can do to change the hose. What helps is to slide your garden hose down the fill port. They sell a short piece of vinyl hose for that, and they work well because it is very flexible, or you can just make your own with an old piece of white water hose but that will be stiffer. Either way, the goal is to keep the stream of water flowing fast through the low point.

There can be a problem with the vent hose, but less likely. If it has a low point, it can be blocked with water sitting there. All it needs is to be blown out. But that's rarely the real problem. Air has to come out obviously, but it doesn't take a large hose to pass a lot of air.