V8 does not use static type inference, as you seem to suggest. It uses dynamic type specialization based on observed types. Only recent builds of SpiderMonkey (not those that are shipping in current Firefoxes) do static type inference.

v8 is not crazy fast. It is a fast dynamic language VM -- beats php, ruby, python and other javascript VMs. In absolute terms, it executes code slowly compared to, for instance, the Java, Scala, C, C++, C# and others.