I have this, always have had... If you put the nozzle all the way in, and squeeze, it clicks off, and takes ages. What i do is take the nozzle nearly all the way out of the neck, and just squeeze, and direct the fuel down the neck that way. Dont have any problems at all then. I know its not the breather on mine, i have an anti-syphon ring fitted in the neck - fitted by previous owner in about 1989, and has been slow since then, unless i take the nozzle out.
Having said that, however, i am still going to check the breather hose!