"Performance" depends on a number of factors beyond the code itself. To take a real world example, immutable operations in Javascript are often more performant

"Performance" depends on a number of factors beyond the code itself. To take a real world example, immutable operations in Javascript are often more performant than mutable operations--thanks to the fact that the Chrome Engine "cheats" with its hot path functionality. However, in Big O, constantly generating new data structures, even within loops, would clearly trash the algorithm's space complexity. On compilers that don't optimize for immutability, the Big O algorithm is more performant; on the ones that do, the immutable approach is more performant. It's because of example like these that you want to disconnect the concept of "performance" from Big O. Because context matters for the former, while the latter relates only to the algorithm itself.