adding fats of low GI carbs to, say whey protein, does not slow down the rate at which your body absorbs the way, it just prolongs the time it takes to digest the entire meal/shake.
this is the analogy I got when it was being explained to me. Say you add a couple of really slow guys to a marathon race. The slow guys are not going to slow down the Kenyans, they'll still finish at their normal speed. The slow guys just slow down the entire time it took to finsih the race. Or something like that