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.
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.
And yes, I still name the company when I tell the story of my worst experience interviewing at a company, 30 years ago.
This way you don't waste the candidate or your time if it's clearly a no after interview 1. They feel a bit bad because they obviously didn't pass, but if you've communicated ahead of time it's not a rug-pull.
I work at a company that grew more than 10x in under 2 years from a couple dozen engineers to hundreds. I personally interviewed well over 100 engineers. My time was valuable, but not so valuable that I couldn't spend an hour productively with each candidate. And certainly not by making them feel like it's an extended failure. On the super rare occasion that someone was gigantic "no", it's still possible to make the candidate feel valued.
One time, I had to shut down an interview cycle because a candidate was abusive to one of my reports. That's the only time I've cut a round short.
Most of the sessions are collaborative solution design exercises. The interviewer can drive the solution, if necessary, and hopefully it's at least educational. The remaining session is a behavioral interview centered on the candidates accomplishments. It's usually possible to set least draw out what the candidate is proud of from their career.
We spent the whole hour interview making zero progress on the task assigned. I kinda wish I had setup some kind of get-out-of-jail situation so that I could have saved everyone the time, but I sat though the whole hour just out of the embarrassment of ending things early.
Despite that, he still managed to make me feel good about the whole experience. At some points in the interview where I was close/slightly off he'd first coax "that's quite similar to X or Y, don't you think?" then if that didn't work he'd coach "here's how X works, elegant explanation, ok, let's talk about Y".
I remember this vividly years later with a smile. Just like I remember all the negative experiences where people were dismissive or ghosted.
Do you mean they can end the interview earlier than 20 minutes, or that it actually takes more than 20 minutes but they can end it at 20 minutes mark?
Either case it doesn't sound good. If they said 20 minutes it shouldn't take much more or much less than 20 minutes.