In reply to mypyrex:
Assuming the downstairs lavatory is fed from the mains then the distance between it and mains supply won't effect the flow in the flexible pipe, assuming there is not blockage or pinching of the pipe.
Is the water supply to the downstairs lavatory OK, ie when flushed does it fill up at a normal rate? If so, then the problem is somewhere in the tap and/or flexible pipe?
Dave