Is it permissible for a worshipper to leave the Imam if he is unable to achieve طمأنينة (tranquility/composure) in his prayer due to the Imam's speed?

It is permissible to part ways with the Imam due to an excuse. If the Imam rushes in his prayer to the extent that it compromises its pillars, and the congregant is unable to follow him, then it becomes obligatory for him to part ways with the Imam and complete the prayer alone. However, if the Imam's rushing does not compromise the pillars of the prayer and does not prevent the congregant from following, then it is not permissible to part ways with him.