It might be because with the double filters, water must pass through two points. More resistance. So the water pressure (and hence speed) is reduced.

In the Panasonic, water just rushes through one filter, so the pressure is maintained somewhat. The distance that the water travels from the tap to the filter's outlet is also much shorter than the double filters.

But the actual filter effectiveness will depend on how good the filter is.. for that, will have to research any labs that might have tested them for effectiveness against 1) bacteria & viruses 2) heavy metals 3) chemicals