[fenomas]

V8 is fast because it tracks how the code behaves and dynamically recompiles functions based on what it sees.

E.g.: "Every time this function has been called so far, the first argument was a small int. Since it's a hot function, I'll recompile it and optimize for that assumption, and wrap the result in a type check. If the type check ever fails I'll fall back to the unoptimized function, and update my assumptions about the function's types."

In other words, a general compiler has to assume all JS types are dynamic. V8 does the same at first, but if it sees code behaving like it was statically typed, it can take advantage of that.