use res.write(buf) instead: http://techblog.safaribooksonline.com/2013/02/22/go-as-an-alternative-to-node-js-for-very-fast-servers/

go-server-result

± for i in {1..10}; do ./wrk http://localhost:8000 -c 100 | grep Requests | awk '{print $2}';done | awk '{s+=$1} END {print s}'
 5329.22

http-serv.go

package main

import "net/http"

func main() {
    bytes := make([]byte, 1024*1024)
    for i := 0; i < len(bytes); i++ {
        bytes[i] = 100
    }

    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        w.Write(bytes)
    })
    http.ListenAndServe(":8000", nil)
}

http-serv.js

http = require('http')
n = 1024*1024;
b = new Buffer(n);
for (var i = 0; i < n; i++) b[i] = 100;

http.createServer(function (req, res) {
  res.writeHead(200);
  res.write(b);
  res.end();
}).listen(8000);

nodejs-server-result

± for i in {1..10}; do ./wrk http://localhost:8000 -c 100 | grep Requests | awk '{print $2}';done | awk '{s+=$1} END {print s}'
9184.36