We have now seen that in order to fulfill the mitzva of eating matza, as well as any other Torah commandment governing eating, one must eat at least a kezayit. One condition must now be added: it is only considered a single act of eating if it is completed within the amount of time it takes to eat half a loaf of bread, or a shi’ur akhilat pras. If one eats half a kezayit, waits ten minutes, and then eats another half a kezayit, it is as if he has only eaten half a kezayit, and hence he has not fulfilled the mitzva (SA 475:6).
In the past, people would bake loaves of bread that would feed one person for a day – half a loaf (a pras) at each of the two main meals.
Our long exile has caused disagreement among poskim over the precise size of a pras: according to Rambam, it is the volume of three eggs, and according to Rashi, four eggs. Several leading Aḥaronim tried to estimate how long it takes to eat a pras, and many views have been offered: nine minutes (Ḥatam Sofer), seven and a half minutes (Arukh La-ner), seven minutes, six minutes, five minutes, and four minutes (R. Ḥayim Naeh and Kaf Ha-ḥayim).
However, all this applies be-di’avad, because le-khatḥila one should eat the matza continuously, and whoever does so certainly fulfills his obligation. In addition, there is no need to look at the clock while eating the matza, because any ordinary person who eats casually but without interruption will certainly finish eating a kezayit within the shi’ur akhilat pras. Even a slow eater, if he does not stop to do other things, will assuredly finish eating a kezayit within a shi’ur akhilat pras, because a pras is eight or nine times the size of a kezayit, and it is inconceivable that one who eats a kezayit without stopping will not finish in the time it takes to eat eight or nine times that amount.
Although some individuals measured and discovered that they could not eat a kezayit of matza within a shi’ur akhilat pras, they only reached this conclusion because they used conflicting measurements. They measured a kezayit according to the most stringent opinion possible – a Ḥazon Ish shi’ur calculated by weight – and on the other hand, they calculated a shi’ur akhilat pras based on the smallest possible measure – Rambam’s shi’ur of three eggs, calculated by volume. These two measurements are contradictory: a Ḥazon Ish kezayit by weight is c. 50 grams, while the three eggs’ volume of matza is c. 54 grams. Since matza is difficult to chew, and a shi’ur akhilat pras is calculated based on the time it takes to eat bread that is easy to chew and swallow, these individuals discovered that they could not finish that kezayit in time. If one wishes to eat two kezeytim, it works out to 100 grams of matza by weight, within the amount of time it normally takes to eat 54 grams of bread; this is impossible. Even according to the Sephardim who do not take the view of Ḥazon Ish into consideration but who measure by weight, two kezeytim by weight work out to c. 58 grams of matza, in the time it takes to eat three eggs by volume, or c. 54 grams. This is impossible during the course of normal eating. Rather, in truth one must calculate the size of a kezayit and the shi’ur akhilat pras using the same standard: if one is stringent regarding the size of a kezayit (and measures it by weight), he must allow himself more time to eat it by calculating the size of an egg based on weight as well, meaning that a shi’ur akhilat pras would be 15 minutes or more.
Therefore, anyone who eats casually, provided that he does not pause or excessively procrastinate in his eating, will certainly be able to finish the kezayit within a shi’ur akhilat pras; thus, there is no need to glance at one’s watch while eating.
Nevertheless, if one paused while eating and did not finish one third of a matza within four minutes, he should act stringently and eat another piece in that time. Logic dictates that since the determination of shi’ur akhilat pras is based on the average person, and since there are so many measurements offered, we should calculate it based on the median of the opinions of the Aḥaronim (6-7 minutes). However, since the mitzva of eating matza is from the Torah, one should follow the strictest opinions. See Peninei Halakha: Berakhot 10:7, 8.
Additionally, according to Minḥat Ḥinukh and Responsa Torat Ḥesed OḤ 32, the shi’ur akhilat pras is determined separately for every food, based on the ease or difficulty of eating it. Accordingly, the shi’ur akhilat pras for matza would be longer than usual, since matza is difficult to chew and swallow. Nevertheless, most poskim who determined a shi’ur akhilat pras understood that it is a fixed standard based on the eating of regular bread.