When printing at high speed, the nozzle will actually experience a wide variety of speed going from 0mm/s to the max speed set on the slicer. The higher the max speed, the wider the range of different speeds. The issue is with the printing temperature being the same, the extrusion temperature will vary dramatically leaving difference surface finishes on the print (from glossy to matte because of a phenomenon called: Shark Skin), High Speed filament reduces these differences by displaying similar surface finish in a wide range of flow rates.
Additionally, high speed filament will extrude much more consistently within the speed range providing a smooth surface whereas regular filament will display holes, artifacts and layers with different thicknesses.
Also, High Speed filament will display far greater overall mechanical properties when printed at higher speed than regular filament because of the more consistent extrusion and the optimized cooling/melting rate of the polymer.