why uglify runing so slow?

[grassmu]

fileUtil.readFile(file, "utf-8", function(err,data) {
if(err) {
console.log("reading file "+ path.resolve(file)+" error");
return;
}
var fileName = path.basename(file, ".js");
console.log("fuck")
var result = uglify.minify(data, {
outSourceMap: fileName + ".map",
fromString: true
});
fileUtil.writeFile(file, result.code, "utf-8", function () {
console.log("file " + file + " created");
fs.rename(file, path.dirname(file)+""+fileName+"_"+util.md5(data, 7) + ".js", function() {});
});
fileUtil.writeFile(path.dirname(file)+""+fileName+ ".map", result.map, "utf-8", function () {
console.log("file " + path.dirname(file)+""+fileName+ ".map created")
});
})

when I run this code, the console diaplay logs very very slow!!!!
when I remove minify code , it runs fast?
please tell me why?
how can i modify it?