Questions about optimizing code for faster results, or how to do faster computations.
Questions on optimizing Mathematica code for higher performance. This may mean faster execution, lower memory usage, or both. Use mathematical-optimization instead for minimization/maximization of objective functions.
I realize that performance-tuning covers more than speed alone. Perhaps every question with that tag should be co-tagged with either memory or speed. (Or some other performance aspect.) If this is what you prefer post it as an answer please. For my proposal see my own answer.