It seems to me that the mishnah berurah in ?? is saying that if the child can eat 1/8 the speed of an adult the dinn aplies. Therefore if an adult takes y amount of time to eat x amount of this food then the tinok must be able to eat x/8 of this food within that time. If the tinok eats matzah and it takes 40 minutes for an adult to eat a “pras” of matzah then the tinok must be able to eat a kizayis within 40 minutes. This is not saying that on pesach if you eat a kizayis within 40 minutes that you have done a halachik “achilah”, since an achilah has to be eaten within a “kidey achilas pras” which is the time to eat half a loaf of bread.
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