It isn’t a turn off when a girl replies too fast to a guy’s message on a dating site. If anything, that kind of speed is a pleasant surprise to the guy. Rather than looking at you as desperate, clingy or overly eager, he admires that you are this straightforward.
He doesn’t have to sit around for days wondering why you haven’t replied to his message. He doesn’t have to fret over the thought of sending you another message just in case his first message never reached you.
Replying to messages fast is actually a very effective way to go about finding what you are looking for sooner. The quicker you message him back, the quicker you learn whether the both of you are romantically compatible.