Some congregations have the custom of making kiddush and having something to eat after Shaḥarit, before shofar-blowing and Musaf. This custom is especially prevalent in yeshivot, where the prayers last until midday or later, and having something to eat enables them to continue praying joyfully and with concentration.
At first glance, this custom is surprising. We know that once the time has arrived to fulfill a mitzva, it is forbidden to eat. For example, on Sukkot we do not eat in the morning before fulfilling the mitzva of lulav. Moreover, on the first day of Sukkot (when the mitzva of lulav is Torah-mandated), if one did not have a lulav available and started to eat, and then a lulav was brought to him, he must stop eating and do netilat lulav immediately (Sukka 38a; SA 652:2). Given this, how is it possible le-khatḥila to permit eating before shofar-blowing?
In truth, the prohibition is to sit down to a proper meal before performing the mitzva, lest one becomes so absorbed in the meal that he forgets the mitzva. When one makes kiddush and eats only a bit, there is no concern that they will forget to return to the synagogue to hear the shofar. Certainly, there is no such concern about a mitzva that everyone anticipates and that people remind one another to fulfill. Moreover, even those who are stringent and avoid all eating may be lenient when necessary. Accordingly, where the prayer service is not lengthy, it is preferable not to stop for kiddush before shofar-blowing.
Le-khatḥila, one should not eat more than and egg’s-bulk (ke-beitza) of mezonot at the kiddush, so that it is not considered a proper meal, but if one is lenient and eats a little more than this, he has an opinion to rely upon. As for fruits, dairy products, and other light foods, more may be eaten. However, a person should avoid eating heavily, as this may make him tired and hinder his concentration. In such a case, eating does him more harm than good.
Many forbid eating more than a ke-beitza because that is the definition of “akhilat keva” (“established eating”) in the sukka (Halikhot Shlomo 2:1; Rav Eliyahu; Ḥut Shani, p. 54). My master and teacher R. Avraham Shapira (cited by R. Harari, Mikra’ei Kodesh: Rosh Ha-shana ch. 7, n. 26) permits eating up to approximately three eggs, which is the amount of a proper meal (kevi’ut se’uda). There is no concern that someone will become preoccupied by his meal if he eats less than the volume of three eggs of mezonot. However, to preserve the solemnity of the day, one should not eat too much.