By performance, I mean "How fast will an optimized $X$ be" or "Is Algorithm $Y$ likely to give good hardware performance?".
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$\begingroup$ The first one is likely to be off-topic because it will change quickly (with CPU architectures) and is difficult to answer, especially if you don't document the optimization level and skill of the programmer. And if you ask for the best possible performance, this is likely to fall under "reference recommendation". The second one is more likely to be on-topic, especially if you ask about side-channel resistant implementations. No guarantees though, this sort of thing depends on the exact question. $\endgroup$– SEJPMApr 10, 2016 at 18:27
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Are questions on crypto performance on-topic?
Yes. There is even a tag for those. However:
"How fast will an optimized $X$ be"
This is not answerable. The only way you can answer is measure an optimized implementation, and even then it could be optimized further. And since performance is platform-dependent, the best way to answer is by the OP to measure on their setup (which is not necessarily useful for others).
"Is Algorithm $Y$ likely to give good hardware performance?"
This is better, but not very objective as stated. Comparing to another (significantly better than Z) would be a possible way to make it more objective. Or having some other clear definition for 'good', I suppose.
What would answerable, objective performance questions be like then?
For example, comparing the performance of an algorithm with different key/field/etc. sizes. Those can often be answered in general, without the particular platform having a significant effect.
Or for something more like your examples it could even look like: "Which MAC algorithm would be faster than X in software, while fulfilling these requirements..." Here the platform could be important information, or it might be answerable in general.
