Is it permissible for congregants to lag behind the imam if he is fast in recitation and prayer, and they are unable to recite Al-Fatiha because of that?

Reciting Al-Fatihah is a pillar of prayer for the Imam, the congregant (ma'mum), and the one praying alone. It is waived for the congregant in two cases: if he joins the Imam while he is bowing (ruku'), or if he joins the Imam just before bowing and cannot complete Al-Fatihah.

It is not permissible for the Imam to rush the prayer in a way that prevents the congregants from completing the pillars and obligations. If the Imam does not respond to advice and continues to rush, and the congregant knows that he will not be able to complete the pillars, he must separate from the Imam and complete the prayer alone.

However, if the congregant is delayed in reciting Al-Fatihah due to the Imam's rushing or due to the congregant's own delay, he should not bow until he completes Al-Fatihah, even if the Imam raises his head from bowing, and then he should follow him thereafter.