Many factors affect the flow rate of water. The size of pipe, the mains pressure, the quality of source water going through the filtration media etc.
It also depends on whether you are filtering just for drinking water or for the whole house.
For drinking water via a filtration system, filtered water will flow at a slightly slower rate than unfiltered water, because the filter media restricts the flow of the water. This restriction helps the filter trap unwanted contaminants. The slower flow rate allows the water more time to come in contact with the filter, thus optimizing the water treatment as the water flows through the carbon filter media and other contaminant-reducing additives. For this reason, it is suggested that consumers use filtered water for both drinking and cooking.
Of course, as more contaminants are trapped the flow rate is affected and this would be a sign to have your filter replaced.
For a whole of house system, although the principle is the same, modern technology allows for much greater tolerance, treatment effectiveness and even automated flushing of the systems for longevity
of media and convenience.