Pirkei Avoth 5:23 Commento: Bartenura, Rambam, Tosafot Yom Tov & Ikar Tosafot Yom Tov

"According to the pain is the reward": According to the greatness of the pain, that you endure in the study of Torah and in the performance of the commandments, will your reward be great.

And Ben Hey Hey said [that] according to that which you are in pain with Torah will be your reward. And they said that only that which you study with exertion, labor and fear from the teacher will endure; but reading of enjoyment and leisure has no endurance to it and [so] no point in it. And they said in the explanation of his statement (Ecclesiastes 2:9), "even (af, which can also be understood as anger) my wisdom remained with me" - the wisdom that I studied with anger remained with me. And because of this, they commanded to place fear upon the students; and they said (Ketuvot 103b), "Throw fear upon the students."

THE REWARD IS COMMENSURATE WITH THE SUFFERING. Rav: as much as one suffers in the learning of Torah and doing of commandments, so much will his reward be greater. This is the reward for the suffering and labor, for if one suffers and labors much the reward will be proportionally greater. But nobody knows the reward for the commandment itself, as the mishna says in 2:1—Maharal in Derech Chaim. See my comments there.