[pcwalton]

V8, like all tiered JITs, doesn't do many optimizations unless the code to be optimized is hot. This is the right thing for V8 to do, because most code on the Web is cold and so it's better to just run the code rather than sitting there optimizing it. But it does mean that there are a lot of optimizations that would be profitable to perform in the aggregate that are nevertheless unprofitable to do at runtime, because the time spent to optimize outweighs the running time of the unoptimized code. AOT optimizers like Prepack can solve this dilemma, by doing expensive optimizations ahead of time so that doing them won't eat into the runtime of the program.

[tadeuzagallo]

That's not all, there's the cost of (down)loading the dead code, and startup cost for loading the unoptimised cost. Even more critical when you consider environments where you can't JIT, like React Native on iOS for instance, which was one of the many motivations for Prepack AFAIK.