gistfile1.js

// IEでも動くように(実質的には弄ったのはtest4のexecのみ)
var r = [];
var test = new Date*1;
for(var i=0;i<10000;i++){
    var src = "http://example.com/lab/test.js"
    src.substring(0,src.lastIndexOf('/')+1)
}
r.push(new Date-test);

var test2 = new Date*1;
for(var i=0;i<10000;i++){
    var a='http://example.com/lab/test.js'.split("/"); a.pop();
    a.join("/") + "/";
}
r.push(new Date-test2);
var test3 = new Date*1;
for(var i=0;i<10000;i++){
    "http://example.com/lab/test.js".split("/").slice(0,-1).join("/") + "/";
}
r.push(new Date-test3);

var test4 = new Date*1;
for(var i=0;i<10000;i++){
    /^.+\//.exec('http://example.com/lab/test.js')[0];
}
r.push(new Date-test4);

var test5 = new Date*1;
for(var i=0;i<10000;i++){
    "http://example.com/lab/test.js".replace(/[^\/]+$/,"")
}
r.push(new Date-test5);

alert(r.join('\n'));