Charlize orgasmed very fast

It is realistic, particularly considering it occurred after weeks/months of building passion.

Secondly, even if you reject that, time is always condensed in movies in one way or another since the filmmakers only have like 2 hours to tell their tale.

I doubt the filmmaker was interested in a drawn out sex scene. Just a little proof they were intimate.
I don't recall the scene specifically, and I love the movie, but seeing CT have an orgasm sounds pretty fun.