What is the ruling on the intrusive thought (waswas) of not joining the second or third congregational prayer due to fear of the imam's speed in bowing and prostrating, which might lead to invalidating the prayer?

There is no justification for the obsessive whispers (waswas). The default is that Muslims pray in the prescribed manner. So, whenever you see someone praying in congregation, pray with them. Be keen to recite the Qur’an and perform dhikr naturally, avoiding indulgence in obsessive thoughts. If the Imam speeds up excessively, preventing the completion of pillars and obligations, do not break your prayer; rather, separate from him and complete your prayer alone.