How can one advise a mosque imam who delays the Fajr and Asr prayers past their prescribed times?

The general principle in the Sunnah (definition) is to perform prayers at the beginning of their time, immediately after the time for prayer begins, except for Dhuhr during intense heat, and Isha if it does not cause hardship to the people. As for Fajr, it is recommended to hasten its performance. If the Imam's delay of Asr prayer causes hardship to the congregants, he should consider their circumstances and gathering, and allow a suitable time between the Adhan (definition) and Iqamah (definition) that accommodates their situation. If the Imam's delay is harmful to the congregants, this can be gently and respectfully explained to him.