coercion.js

String.prototype.method = function() {}
function func(str) {}

var stringObject = new String("hello world");
var stringPrimitive = "hello world";

exports.compare = {
  "stringObject.method()": function() {
    stringObject.method();
  },
  "stringPrimitive.method()": function() {
    stringPrimitive.method();
  },
  "func(stringObject)": function() {
    func(stringObject);
  },
  "func(stringPrimitive)": function() {
    func(stringPrimitive);
  },
};

require("bench").runMain();

coercion.js.out

benchmarking /Users/marcel/coercion.js
Please be patient.
Scores: (bigger is better)

stringObject.method()
Raw:
 > 54363.63636363636
 > 51750.24975024975
 > 53206.793206793205
 > 54864.13586413587
 > 54911.08891108891
Average (mean) 53819.180819180816

func(stringPrimitive)
Raw:
 > 38549.45054945055
 > 44656.343656343655
 > 44731.268731268734
 > 44581.418581418584
 > 43761.23876123876
Average (mean) 43255.944055944055

func(stringObject)
Raw:
 > 39459.54045954046
 > 43741.25874125874
 > 44738.26173826174
 > 43252.74725274725
 > 44784.21578421578
Average (mean) 43195.20479520479

stringPrimitive.method()
Raw:
 > 2061.9380619380618
 > 2216.7832167832166
 > 2217.7822177822177
 > 2215.7842157842156
 > 2212.787212787213
Average (mean) 2185.0149850149846

Winner: stringObject.method()
Compared with next highest (func(stringPrimitive)), it's:
19.63% faster
1.24 times as fast
0.09 order(s) of magnitude faster

Compared with the slowest (stringPrimitive.method()), it's:
95.94% faster
24.63 times as fast
1.39 order(s) of magnitude faster