On paper this shouldn't be the case, The Morrflate TenSix has a rated 10.6 CFPM output, but the Schrader valve's are usually 2 CFPM or less, so they become the bottleneck. If you are pumping one tire, you'll be limited by the Schrader valve to only 2CFPM, but if you are pumping 4 you should be able to pump up at 8CFPM. So in theory, those hose splitters should be able to pump four tires up in the same time it takes to fill one.
I say in theory, because that is not at all what I have experienced using a four-tire kit with my double-ARB. It definitely felt faster, but no where near 1/4th the time.