I think that was what YorkshireDave was on about with his comment about the diaphragm washer deforming - they tend to stretch towards being in the closed position when relaxed so it is only the water pressure to push them open, and if the water pressure isn't very high, flow could then be poor, whereas the new one will sit in a more neutral way and so allow a better flow. I would hazard a guess that your valve was working on tank pressure and that what was wrong with your valve was just the washer (or you had tank pressure and the high pressure (white) nozzle fitted)?
You see, I am always very reluctant to just replace something and call it 'broken', because if I can't mend something and can't work out what is wrong with it either, I feel I have failed to learn anything, so I always try to work out what has gone wrong. Sometimes, I waste a lot of time, and sometimes I save myself a fortune. But I enjoy it