What is the ruling on the prayer of one who lags behind the imam in bowing and rising from it, due to completing the Fatiha or the imam's extreme slowness?

The questioner is warned against obsessive thoughts (waswasah) and advised to act upon the easiest opinions to repel them. There is no harm for the questioner in acting upon the opinion of those who do not consider the recitation of Al-Fatiha obligatory for the congregant (ma'mum). His prayer is valid in the first scenario because he raises (from bowing) after his imam, and his raising before the imam finishes saying "Sami'a Allahu liman hamidah" (Allah hears him who praises Him) does not harm. As for the second scenario, his prayer is also valid according to many scholars.