Basically, modern JS engines are fast because they do a lot of inferring about types. They do this both statically (by inspecting the code at compile time) and dynamically (by watching the code as it executes), and they do it for objects as well as literals.

For a hand-waving theoretical example, if you have code like:

    var a = { foo:1 }
    var b = { foo:2 }
    var c = { foo:3 }
    doSomething(a)
    doSomething(b)
    doSomething(c)

then modern JS engines can see that the "doSomething" function always gets called with arguments of the same type signature, and if it's a hot function they might optimize the function around that information.

If you want to know more, searching on "V8 hidden classes" should turn up relevant articles.