Even before most musallis finish reciting Tashahhud, the Imaam makes salaam.

Answer

If the Imaam had recited Tashahhud, even at his uncalled for haraam speed, then the Salaat is technically valid in terms of the rules of Fiqh.

However according to the reality of the Sunnah, the Salaat as described by you was not valid. Such a decrepit Salaat is wrapped up like a dirty cloth and flung at the performer by the Malaaikah who prevent the damaged Salaat from proceeding into the heavens.