Is it obligatory to submit with the Imam even if the worshiper is unable to complete the whole Ibrahimiyyah Prayer in the last Tashahhud due to the Imam's speed?

The congregant must follow the Imam in all actions of the prayer, even if the Imam prays quickly, because the Prophet (peace be upon him) said: "Indeed, the Imam is appointed to be followed."

If the Imam says the salām after the congregant has completed the tashahhud (definition) but before reciting the salat on the Prophet (peace be upon him) and the supplication in the final tashahhud: - According to the Malikis: The congregant should say the salām immediately after the Imam, because reciting the salat on the Prophet (peace be upon him) is considered a sunnah (definition) for them, and it is disliked for the congregant to remain to complete the supplication after the Imam has said the salām. - According to the Shafi'is and Hanbalis: Reciting the salat on the Prophet (peace be upon him) is obligatory, and the obligatory part of it is saying: "Allāhumma salli 'alā Muhammad" (O Allah, send prayers upon Muhammad). Anything beyond this is sunnah. Therefore, the congregant must only perform the obligatory part of the salat on the Prophet (peace be upon him) and say the salām immediately after the Imam without delay, and not engage in any sunnah action or supplication.