What do you change if it's clear that the candidate is completely incapable of doing any of those things? Do you cut it short? Talk them through it step-by-step? Unfortunately I've found that not all candidates who get through a recruiter can do what they say they can, so I keep a question at the very start that eats up maybe 5-15 minutes (sometimes less) for a good candidate, but some very bad candidates spend a full 45 minutes on it and don't finish.
The interview will be very short once you've gone through your questions, that's all.
For your 5-15 minute question, I don't know the nature of it, but why don't you just move on with reference to time? I'm sure you have a lot of other questions you should ask. It is on the interviewer to be the timekeeper of the interview.
I was curious about the parent commenter's approach with the sorts of questions they ask as they are more open-ended, but also somewhat knowledge-gated. I was interested in any potentially unique techniques they employ. However, I was not looking to specifically fix anything with how I conduct my interviews.
My first question is a very specific coding question and it's a weed-out question. So if someone doesn't figure it out, they've been weeded out, simple as that. I ensure that candidates don't waste time figuring out nit-picky edge-cases and give appropriate hints when they're utterly stuck, but the question does its job very well.
