From: JSON API for some bank account
I can see questions where somebody has designed code to be explicitly fast and needs another pair of eyeballs to validate the code.
But are questions that ask explicitly about how to get more performance on-topic?
In my opinion the implied question in all posts on this side should be: "What can be improved about this code?". The asker is free to specify that he's particularly interested in suggestions about the code's performance (the same way he'd be free to say he's particularly interested in getting rid of the repetition in the foobar method), but any aspect of the code (be it the overall design, the coding style, the names of the variables or the performance) should be fair game for the answers.
If a question specifically restricts the answer to a particular area like performance and is not interested in a general code review, it is IMHO too narrow in scope for this site. In this case the OP should either be asked to make the question more general (i.e. ask for any suggestions, especially performance related, instead of only performance improvements) or ask on stack overflow.
