1. Is the Line to the toilet a standard Plastic line or a flexible rubber/vinyl line?
2. Does the line if flexible go into a floor or wall opening? That line is usually either "t"d off or runs behind the wall or floor to its cold water connection point. IF flexible it could be bent/crimped inside the wall causing restriction.
3. Another cause is the toilet water line inlet has debris/calcium build up restricting the normal flow of water. To fix this you pull the toilet(easy to do) and look at the inlet and blow out if you see debris. Doug
