The possible syntax for Object literals is a lot more complicated than what JSON can contain. Hence the JSON parser is faster than the JS parser for the same object.

JS object literals allow such fun things as:

  {
    "foo": "bar",
    foo1: function () { return "bar" },
    foo2: () => "bar",
    foo3 () { return "bar" },
    get foo4 () { return "bar" },
    ["foo5"]: () => "bar"
  }