Not every code is written with performance in mind. In fact, most aren't. This alone gives much leeway for dynamic languages; if you can make more performant program easier to write, they can pay off even when their maximum performance is limited (this is my definition of "reasonably performant"). The exact amount of leeway will of course depend on circumstances but I guess 10x is a fair estimate, which many JIT implementations can reach---not just crazy beasts like JS engines.