bottom fill wc cistern?

have a bottom fill cistern, and it fills up but takes
20 mins, its like hissing out its one of the new type cistern valves, local plumbing shop say buy new internals, but as its only 2 years old want to see if I can fix it, does it come apart, and would it be something simple as a washer, or hard water build up, theres a knurled locknut butting onto the unit, which has the rise and fall float, water levels ok but takes 20 mins, or is this planned obsolescence. ie not user friendly and just replace unit.