Node js slow performance on 60 connections

Don't connect to memcached on each call, instead reuse an existing connection:

var app = require('express')()
  , server = require('http').createServer(app)
  , io = require('socket.io').listen(server);

var memcache = require('memcache');

var client = new memcache.Client();

server.listen(3005);

client.on('connect', function() {
    console.log('connect');

    io.sockets.on('connection', function (socket) {
        socket.emit('connected');

        socket.on('check',function(data){
            client.set(data.id, 1, function(error, result){

                console.log('Key added');
                console.log(result);

                // maybe unnecessary, just check if error is present
                client.get(data.id, function(error, result){
                    socket.emit('checked',data.id);
                });
            },600);
        });
    });
});

client.connect();

You call:

client.connect();

but your corresponding connection handler merely logs a message rather than performing the action you wanted to perform.

client.on('connect', function(){
  console.log('connect');
});

You have a function:

client.set(data.id, 1, function(error, result){
  console.log('Key added');
  console.log(result);
  client.get(data.id, function(error, result){
    socket.emit('checked',data.id);
  });
},600);

.. which really should be wrapped inside the client.on("connect") handler so I wonder why your code succeeds at all?

Possibly your first or first few attempts to access memcached fail but then succeed because the client variable is re-used and the connection may stay alive. But - risky.