Ijtimai Du'a and naseehat after Salaah

After the Fardh Salah the Imam makes Du'a in which many if not the majority of people participate, arguably making it an Ijtimai Du'a. Thereafter, the Imam stands up and gives naseehat to the assembled congregation, thereby delaying the Sunnah prayer. From the end of Salah to the end of the naseehat is between 4-5min. It may be contended that nobody is obligated to sit for the naseehat. However when the majority, if not all the people sit for the nassehat, it makes it
awkward for one or a few individuals to depart to perform their Sunnah as:

(a) People are in the way and get inconvenienced.

(b) Persons draw uncomfortable attention to themselves by getting up and leaving.

Is delaying the Sunnah for this duration or at all permissible?

If not, what is the proof that such a delay is impermissible?