server.js

var http = require('http'),
    fs = require('fs')

var start = +new Date

var data = 0
var last_calculation = 0
var written = 0

var calculate = function() {
    var elapsed = new Date - last_calculation;
    console.log('speed', data/elapsed + ' kBps')
    console.log('data', data/1000 + ' kB')
    console.log('sum', written/1000 + ' kB')
    
    data = 0
    last_calculation = +new Date
}

http.createServer(function (req, res) {
    var socket = req.connection

    res.on('drain', function() {
        console.log('drain', new Date - start);
    })
    
    // Proxying write
    var write = res.write;
    res.write = function(buffer) {
        data += buffer.length;
        written += buffer.length;
        
        var flushed = write.apply(this, arguments)
        console.log('writing', buffer.length, socket.bufferSize, flushed, new Date - start)
        if (!flushed) calculate()
        ;
        return flushed
    }
    
    // Proxying end
    var end = res.end;
    res.end = function() {
        console.log('end')
        return end.apply(this, arguments)
    }

    res.writeHead(200, {'Content-Type': 'text/plain'})
    fs.createReadStream(process.argv[2]).pipe(res)
}).listen(1337, '127.0.0.1')

console.log('Server running at http://127.0.0.1:1337/')