That is extremely unprofessional and a bad look for the interviewer. I have done over a 100 interviews including some really bad ones, never have I ever cut one short. When you are interviewing you are representing the company, whenever I interviewed someone even if I knew it was going to be a clear no, I tried to make them feel good about the process as what that person is going to go out and say directly reflects on the company and future hires. 20 mins of your time is a drop in the bucket in terms of impact of getting a bad reputation, not only are you potentially shooting yourself in the foot with other candidates but leaving a bad taste in the mouth of the one you are interviewing who might in the future would otherwise reapply at a different stage in their career if/when they become a stronger candidate.