To be fair, if it's part of the lines in a macro, it's going to be read and executed regardless, so complaining about a /micon line taking time is about as useful as complaining about /echo lines also taking up time (Hint: they do). Any lines you put into a macro will have macro time. To expect a /micon to not have macro time is just foolishness.

And in any case, it's only 30ms or so. Not game-breaking enough to be a real big difference.