Why is the cistern filling up really slowly?

Sounds like airlock or the washer has partially stuck on the inlet restricting the flow in, this mainly happens on low pressure ie: attic tank fed ones.
Turn off the water to the toilet cistern if a plastic unit at the water inlet valve end of the arm is pin with split pin holding the float arm on the valve assembly. Remove the spit pin then pin then lift the arm clear. Undo the large nut on the inlet valve draw it off over the plunger sticking out the end then pull the plunger if not off already away from the large washer pull washer away from inlet tube get new washer and replace in reverse. Drop plunger into the nut with flat disc side inward then lay washer over top of plunger carefully refit the nut complet with plunger and washer. Replace the arm job done when water turned on. If still not flow check you have water in the attic feed tank. If it's mains fed the same goes the washers are the same.

bob
May 2009