Personally, I feel questions about performance are on topic on Code Review. This topic makes me believe most people agree. As an example: this question.

Shouldn't "Performance" be added to the list of questions you can ask here in the FAQ?

Perhaps to prevent "micro benchmarking code golf" as mentioned by Jeff, a bit more clarification could be added. However, I don't think people will misunderstand it, since above this list of valid topics is mentioned in bold: "sharing code from projects you are working on". This excludes optimizations 'just for fun'.