Does excessive speed when returning to bowing—after rising from it and then remembering to say: "Allah hears those who praise Him"—affect the obligatory tranquility (ṭumaʾnīnah) in prayer?

The place for saying "Sami' Allahu liman hamidah" (Allah hears the one who praises Him) is between starting to rise from bowing (ruku') and standing upright. Returning to it after its proper place has passed is incorrect because it is a Sunnah (recommended act) while bowing is an obligatory act (fard). One does not return from an obligatory act to a Sunnah. Prostration of forgetfulness (sujud as-sahw) suffices you. If the return was intentional, the prayer is invalid and must be repeated. If it was due to ignorance or forgetfulness, the prayer is valid. The saying: "Sami' Allahu liman hamidah" is a Sunnah, and abandoning it does not invalidate the prayer.