slow flush cistern?

if water supply pressure to the rest of house taps is o k then check the float arm is freely dropping when the toilet is flushed the arm can stick to the cotter pin rotate the cotter in its hosing to see if free.
lastly the inlet valve could have a blockage at the back of the shutoff valve(rubbish that has come through the water supply pipe) valve needs stripping to clear usually a chuck of grit

wirecutterman
April 2009

There is likely to be a partial blockage in the fill valve - unless the water pressure has significantly decreased somehow. These can be dismantled and new washers fitted, but I just put a complete new one on - I use a Torbeck type most of the time.

John
April 2009