Disagree on this one. In an interview there is no "the answer", it's a dialogue. I've interviewed a lot of people, often using performance related questions, and trust me, there are lots of candidates whose only answer to those is "add a cache", even after multiple follow-up questions or hints like "is there anything else that can be done?", "try thinking outside the box", "what can be done with the database itself", etc. Only a novice interviewer will be fooled by the first answer. If you cannot demonstrate more solutions after that, it shows that you clearly have no experience or problem-solving ability, which is the whole point of the interview to find out, not whether you have studied through a set of common questions.

btw, "scale up" is the second most common answer from those who can't provide better solutions. :)