Thanks to the author from meteor-router, I was able to fix the problem. I was using fiber the wrong way. As described at https://github.com/laverdet/node-fibers#futures, it's not recommended to use fiber without an abstraction between your code and the raw API.
Fortunately, fiber provides one abstraction called future which can be used for my use case! Here is the working code:
var require = __meteor_bootstrap__.require,
Future = require('fibers/future');
Meteor.startup(function() {
Meteor.Router.add('/image/:hash', function(hash) {
var response = this.response;
var fut = new Future();
response.writeHead(200, {'Content-Type':'text/plain'});
setTimeout(function(){
response.write("hello hello");
fut.ret();
}, 1);
fut.wait();
});
});