Bo

As 'ה tells Moshe what He's going to do to the first born children in Egypt, He tells him that the Jews will need to put blood on their doorposts so that 'ה will pass over the Jewish houses. Because of that mitzva of putting the blood on their doorposts, Jews will commemorate that great miracle leading to the exodus from Egypt by having a seven day holiday where they don't eat bread but eat Matzos instead.

Then, the Torah continues ושמרתם את המצות that you should watch the Matzos, which means, on face value, not to let the Matzos become חמץ.

Rashi , quoting the מכילתא, says רבי יאשיה אומר אל תהי קורא את המצות אלא את המצוות , don't read it as the Matzos but rather read it Mitzvos , כדרך שאין מחמיצין את המצות כך אין מחמיצים את המצוות, just as we don't let the Matzos become leavened so too don't let the Mitzvos become leavened by neglecting the opportunity to do them. When the opportunity to do a mitzva comes your way, do it without delay.

The כתב סופר asks if we’re going to say that the posuk is talking about doing the Mitzvos without delay, how does that relate to the rest of the posuk when it says כי בעצם היום הזה הוצאתי את צבאותיכם מארץ מצרים  because on that very day I took you out of Mitzrayim. This shows that the posuk is talking about going out of mitzrayim.

He explains by way of the Midrash that says had the Jewish people stayed in Egypt even one more second, they would not have been able to get out. They would have sunken to the fiftieth, lowest level of impurity from which there was no return. This demonstrates the importance of each second. This is an optimum place to learn to do the mitzvos without delay for had 'ה delayed taking us out it would have been too late. So, too, with Mitzvos if we don't do them right away, it might be too late.

Based on this, one might think that Mitzvos should be hurried & rushed so that they get done right away. To counter this, the חפץ חיים says we read the word ושמרתם as in שמר את הדבר , he waited, that the way to do Mitzvos is calculated, thoughtful with a clear mind to know that you’re doing it for the Mitzva & not the fanfare. Don't delay the start but don't rush through it or do it for the hype.   

The following story shows how Mitzvos should be done. It was known that ר' חיים מצאנז was very meticulous with the Mitzva of מרור & would take the strongest bitter herb he could find and eat the maximum amount to really feel the pain. One year, when he was older and his strength had weakened, the doctor warned him that  eating  מרור like that would be detrimental to his health.

When ר' חיים מצאנז was at his seder, he told his attendant to bring him the מרור like he does every year. When it came time to eat it, he took his regular big כזית, held it in his hand & made the beracha אשר קדשנו במצותיו וציונו ונשמרתם מאד לנפשותיכם , who sanctified us with His commandments & commanded us to watch our health.

And then he returned the מרור to the plate.