Modern compilers are extremely clever and will produce machine code that takes full advantage of modern CPU branch predictors, and reorder instructions to better take advantage of pipelining. This in itself will make the same code run at different speeds depending on the input data.

Then there is the whole issue of compiler version roulette. As a developer you have no idea which version of compilers your users and distros will use, and what new and wonderful optimisation they will bring.

Within a version, yes, but not cross version. Different versions of GCC/Clang etc can give you completely different code.