You can do the experiment, if you like. Get a handful of friends together, and a mess of dice . Each person will do two sets of ten rolls, each roll will be timed - one set is "roll 8 dice, and add them up" the other is "roll 8 dice, tell me how many are 5 or above". You can use anything d6 or higher - if you want to make the issue most clear, use d20s.
You will very probably find that the set of "roll and sum all" will be slower than the "roll and count hits".
When you are summing, for each die you are adding a number between 1 and N to the total, where N is the number of sides to the die. When you are counting hits, for each die you are adding 0 or 1 to a total. The latter is just faster for most people.