Most of the code in any program is not performance critical. Yet the reason that this is the case is that most of the time of any program is spent in tight loops. Those tight loops are what must be optimized. And that is why performance-minded systems programmers use top-notch optimizing compilers.

The popular dynamic languages have gotten away with being relatively slow because programs written in them are either not CPU bound or spend most of their CPU time in C libraries. This works great for them -- dynamic languages are awesome! But the calculus changes when you're programming in a language that defines the entire stack. When there's no C-compiled code to fall back on, micro-optimizations start to matter.