Learn Portuguese (Portugal): Não sejas impaciente, porque a contração está para começar. - Don't be impatient, because the contraction is about to start.

The structure estar para + infinitive highlights that an action is imminent. While vai começar just means it is going to start at some point in the future, está para começar stresses that it is on the verge of happening right now (about to start).

Could I say está prestes a começar instead?

Yes! Both estar para and estar prestes a followed by an infinitive mean to be about to do something. They are interchangeable in this sentence, though prestes a can sometimes feel slightly more formal or add a tiny bit more urgency.

Why is the verb form sejas used instead of és?

To give a negative command (telling someone not to do something), Portuguese uses the present subjunctive form of the verb. Sejas is the informal you (tu) form of ser in the present subjunctive. If you said não és, you would just be stating a fact: you are not.

Why use ser (não sejas) instead of estar (não estejas) if being impatient is just a temporary feeling here?