Is it permissible to pray behind a mosque imam who does not recite well and prolongs the prayer?

The Imam should lighten his prayer in consideration of the people's circumstances. This is supported by the Prophet's saying to Mu'adh: "Are you a trial, O Mu'adh?"

The questioner should advise the Imam with wisdom and gentleness regarding lightening the prayer. If he does not respond, there is no objection to praying behind another Imam. A good voice is not a condition for leading the prayer, though it is preferable to prioritize someone with a good voice if they are equal to others in memorization and proficiency.