You must follow the Imam
Question :
I performed Zuhr prayer behind the Imam, but in the last two Rak'ahs, I was unable to complete the recitation of Fatihah Al-Kitab, since had I completed it, I would have missed the bowing with the Imam. Should I complete the recitation, or bow and the recitation of the Imam will be sufficient for me?
Answer:
It is apparent that the Imam is rushing the recitation and shortening the standing, or that you are slow in reciting, prolonging the prononciation and the enunciation of the letters. If it is the former, you must advise this Imam against haste and rushing which causes the worshippers to be unable to perform the pillars (of prayer). If it is the latter, then you must be briefer (in your recitation) and implement whatever speed is necessary for you to perform the pillars with the Imam. In any case, you must follow the Imam, even if you are not able to complete the recitation, if you fear that you will miss the bowing.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 198-199