The interviewer controls the use of time, so don't worry about imposing a burden. In my case, anyway, I'd rather have a fascinating technical conversation than ask my usual programming challenge and algorithm design questions, so your great question might seem more like a gift than a burden. I'm serious! With me, anyway, ask big questions like "what should/will HPC hardware and programming languages look like in 10 years?" and then tell me why you think my answer is crazy.