No, but it saves you having to emit machine code that does type checking and dispatch at run-time. You need type information if you want inner-loop performance that is competitive with languages (like C) that are designed for efficient compilation.

The generated code for something like "a = b + c" is very, very different if you know at compile-time whether b and c are strings or numbers or something else. Modern JavaScript VMs go to a lot of effort to guess or infer type information before JIT compilation.