Long response time with static express files

I've a simple Express.js app with some static files served by Express.js itself.

When I open the index.html page it tries to load the bundle.js script but it takes ages, it does not happen everytime but only sometimes without any evident reason...

It happens with any kind of file, sometimes js, sometimes css.

My app is something like this:

var app = require("express")();
app.use("/", express.static(__dirname + "/public"));
app.use("/bower_components", express.static(__dirname + "/bower_components"));
app.listen(3000);

I'm running the app locally.. why is there this problem?