This isn't really fair: some implementations really do rule out certain optimizations. Turing-Completeness only refers to what is computable, not what actual algorithms and optimization techniques are possible.
And to be more specific, Turing completeness refers to what is computable with an unbounded amount of memory and time. This is why we have complexity theory layered on top of computability theory. Some things are theoretically computable but practically infeasible. Compiler optimizations can help greatly in some of those cases.