EDIT: The PBS NOVA special "The Day the Dinosaurs Died" describes the mechanics of the Chicxulub impact starting at about the 33 minute mark. It's as instantaneous as one would intuit, given the magnitudes involved:
> NARRATOR: The asteroid had punched a hole nearly 20 miles deep, gouging a crater 124 miles wide. Over 40,000 cubic miles of rock were displaced, rising into towering mountains.
> JOANNA MORGAN: So, this is an absolutely amazing event, because that happened in less than two minutes, and if you think of it, mountains the size of the Himalayas were formed in seconds.
> NARRATOR: As the mountains collapsed, they formed what's known as a "peak ring," found only in the largest of super craters. It's the only peak ring left on Earth; the next nearest is on the moon.