Hm, sometimes when I’m interviewing someone who I know is a “no” it feels like I’m leading them on if I go through the full interview. I thought it would be more respectful of their time to let them know that early, but maybe I am wrong about that?
I’ve also heard (via HR) about candidates being surprised to not get the offer in cases where I kept the interview going and pretended things were going well.
The interviewee already has their schedule booked off during that window, that extra 20 mins is not saving them any time. If your concern is their time, I would assume they'd "waste" more time stewing on an early interview exit and thinking about it rather than if it just ended normally.
Finish the interview, and provide feedback if asked why it was a 'no'.
Interviewing isn't just for fun, and even if you don't get the job, you can go through the interview to practice, and hopefully to figure out what went wrong.
If after 20 minutes they just say 'I think this is a no so go home' I wasted a lot more than 20 minutes and got nothing out of it.
I’ve also heard (via HR) about candidates being surprised to not get the offer in cases where I kept the interview going and pretended things were going well.