Q. Why does Rashi mention that we learn on “ushmartem es hamatzos,” that we should do a mitzvah immediately, we know that already from “vayashkem Avrohom baboker?”

A. On the posuk; Ushmartem es hamatzos,You shall watch over the matzos. (12; 17). Rashi quoting Mechilta mentions Rabbi Yoshia saying: Do not read: matzos rather read mitzvos. Just as we may not permit the matzos to become leavened, so may we not permit the commandments to become fermented [i.e., to wait too long before we perform them]. Therefore, if a mitzva comes into your hand, perform it immediately. Similarly, on the posuk (Bereshis 22: 3); Vayashkem Avrohom baboker. Avrohom arose early in the morning, we learn that zrizin makdimin lemitzvos; quickness and alacrity are esential. (Megila 20b)
It is quoted on the name of Moreinu Horav Moshe Feinstein zt’l that there are two kinds of mitzvos. Those who do not have a definite determined time for their observance, such as building a suka. The Rema (625: 1) mentions that it is a mitzva to begin making the suka immediately after Yom Kipur since; we do not permit the commandments to become fermented. However, mitzvos that have a set exact time to comply with them, such as bris-milah on the eight day or blowing shofar on Rosh Hashana, one should start at the very beginning of the day or time to comply with the mitzva. (Orach Hamoadim p. 535).
Horav Shlomo Miler’s Shlit’a opinion is that; zrizin makdimin lemitzvos applies to the mitzva before one begins complying with it, while Ushmartem es hamatzos or not allowing the commandments to become fermented, applies at the time when one is already performing the mitzva.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a.