The phrase "most quickened" refers to something that has been accelerated or stimulated to a great extent. The antonym for "quickened" would be "slowed down," indicating that the pace of the activity or process has been reduced. Other viable antonyms for "most quickened" include "calmed," "pacified," "soothed," and "relaxed". Rather than being stimulated or energized, something that has been calmed is placated or calmed. It's important to keep in mind that depending on the context, multiple antonyms could apply. Therefore, selecting an antonym that is most fitting for the context in which it is used would be effective.