All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The Imam should initially wait for a short period before starting the class to enable people to finish the Athkaar due after the payer, and to enable those who came late to finish their prayers without being distracted. Once they finish, then he can start delivering his class, but he should not raise his voice too loud and should keep it at a moderate level.
If the Imam knows that people will leave if he does not start the class immediately, then there would be no harm in starting the class as soon as he finishes the prayer. This is because hastening to start the class in this case would achieve the benefit of spreading knowledge and calling people to Allah. In such a case, there is no harm if one finishes the Athkaar after the class ends, and this delay is not considered a religious innovation (Bid’ah) as stated by Ibn Uthaymeen .
Allah knows best.