Is the prayer of one who delays joining the Imam in order to complete the remembrance (dhikr) valid and sufficient, or must it be repeated? And is it permissible to pray behind a fast-reciting Imam?

If the Imam's speed does not compromise tranquility and he fulfills the obligatory acts, then his prayer is valid. Otherwise, it is nullified, and both the Imam and the congregant must repeat it. Following the Imam is preferable, even if it entails omitting an obligatory act, such as the glorification (tasbih) in bowing, or even omitting a pillar to follow the Imam. A congregant's delay behind the Imam to perform the glorification (tasbih) in prostration does not invalidate the prayer if he performs what he has fallen behind on and catches up with his Imam. The glorification (tasbih) in prostration is a Sunnah, and its omission does not invalidate the prayer.