Duck typing vs. instanceof ... script

gistfile2.txt

var repeat = 100000;

// 1a) Duck typing - true
var t = +new Date(),
    obj = [1,2,3];
for (var i = 0; i < repeat; i++){!!(obj && obj.concat && obj.unshift && !obj.callee);}
console.log(+new Date() - t);

// 1b) Duck typing - false
t = +new Date(),
    obj = {};
for (var i = 0; i < repeat; i++){!!(obj && obj.concat && obj.unshift && !obj.callee);}
console.log(+new Date() - t);

// 2a) instanceof - true
t = +new Date(),
    obj = [1,2,3];
for (var i = 0; i < repeat; i++){obj instanceof Array;}
console.log(+new Date() - t);

// 2b) instanceof - false
t = +new Date(),
    obj = {};
for (var i = 0; i < repeat; i++){obj instanceof Array;}
console.log(+new Date() - t);

Result

FF 3.6b3

285
200
191
192

GC 7.0

245
178
184
182

Safari 5.0.2

56
37
32
32