who told you it slows down the air velocity? the engine sucks in pulses and air itself has mass, the airbox and manifold have plenum design for a reason.. Dont let the parfarmanss dukaan tell you otherwise.
The "parfarmanss" you feel is nothing but the loud sound of the air being sucked.
btw - if you remove the air filter your engine will suck on dust etc which will grind away the cylinder walls, mix with oil to ruin the bearing surfaces and render your engine junk.