Earlier quoted context omitted.
I've had interviews with startups that will use a quick interview turn around time as a selling feature to close candidates. I've also interviewed at Two Sigma where I was flown in onsite and by the last interview they had already reviewed most of my feedback and told me that they were going to extend an offer. I got the sense that if I wasn't using a 3rd party recruiter with rules they might have handed me an offer…
I had a similar, but inverse experience - was brought in for an all-day interview, and after half a day both they and I realized that I wasn't going to be a good fit, so they let me go. No hard feelings on my end; I was out of my depth at the time, and knew it, and was glad I wasn't going to have to waste the rest of my day and theirs.
If, during an interview, it becomes obvious to you that you aren't a good fit at the place for whatever reason, you don't have to wait for them to end the process early.
Doing so yourself is not only acceptable, it's also a professional courtesy that they will appreciate.
Remember, with all interview processes, you're evaluating the potential employer just as much as they're evaluating you.