Haste is undesirable

خُلِقَ الْإِنسَانُ مِنْ عَجَلٍ (Man is made of haste. - 21:37). عَجَل ('ajal) means haste or hurry. The word is used in situations when one desires things to happen before their time, and this trait is bad by its very nature. In another place also the word is used to denote human weakness. For instance وَكَانَ الْإِنسَانُ عَجُولًا i.e. Man is prone to haste - 17:11, meaning that he is very impatient. When Sayyidna Musa (علیہ السلام) went to the mount Tur in a hurry leaving his people behind, he was censured by Allah Ta` ala.

Prophets (علیہم السلام) and the devout people who try to excel each other in the performance of righteous deeds have been commended and their zeal to do good deeds does not constitute haste and hurry because they do not try to do these deeds before their time. In fact they do the deeds on time, but try to excel each other in quantity and quality.

Here خُلِقَ الْإِنسَانُ مِنْ عَجَلٍ (Man is made of haste - 21:37) means that haste and hurry is one of the many weaknesses which are inherent in human nature. When a man is identified by some intrinsic trait of his character, me Arabs used to say that he is 'made of that trait. For instance a short tempered man would be called a 'man made of anger'.

سَأُرِ‌يكُمْ آيَاتِي فَلَا تَسْتَعْجِلُونِ (I shall show you my signs -.21:37) Here the word آیات (signs) refers to those miracles and events which bear evidence to the honesty of the Holy Prophet ﷺ and the truth of his message. (Qurtubi)

These miracles also occurred during the battle of Badr (غزوہ بدر) when the Muslims who were considered weak and worthy of contempt gained a great victory over their enemies.