yeah at a guess in the fish shop they had whopping great big industrial sized RO units, yours is like the home version, they are much much slower, your results are not at all surprising, we had the same sort of speed with a 50GPD unit.
also remember the temp fo the water will have an effect on teh speed, if it's too cold as is likely at this time of year it slows it all down although helpfully i can't remember why.