Answered by Shaykh Amjad Rasheed
Is it disliked for a follower to lag behind his imam by prolonging his bowing (ruku’) and prostration (sujud)?
In the Name of Allah, Most Merciful and Compassionate. May His blessings and peace be on His Beloved Prophet, the best of creation, and his family, companions, and followers
If he lags behind such that the imam raises his head from bowing and begins going down for prostration, then it is disliked. Similarly, it is disliked if he lags behind such that the imam raises his head from prostration, sits, and begins going down for the second prostration. If, however, he prolongs the bowing and prostration such that the imam remains in the integral that immediately follows the bowing (i.e., the standing up after bowing) or in the integral that immediately follows the prostration (i.e., sitting or standing), then it is not disliked.
– Amjad
(Translated by Hamza Karamali)