Node.js, cluster, express, apache ab and mongodb

I'm new to mongo and have a few performance tests setup with Node.js (v0.4.12), cluster (Learnboost v0.7.7) and express (v2.5.9). I'm trying to connect to mongo and do a simple performance test with apache ab. I've set this up with MySql and it works flawlessly, but when I try to do the same test with mongo I don't get the results I expect.

With MySql, I send 10,000 requests to the node.js cluster (8 cores) and I get 10,000 rows inserted into the db. No issues. But, when I do the same thing with mongo, I only get around 4,500-4,900. I've tried mongoskin, mongo-lite, node-mongodb-native and all have exactly the same result.

I thought maybe it was the pool size, so I increased/decreased that. Nothing. No errors or exceptions. I'm at a loss...

Any help would be appreciated.

Thanks.

The code for the test is below;

app.js

var mongoLite = require('mongo-lite');
var express   = require('express');

var count  = 1;
var config = {
host    : 'localhost', 
user    : USER,
pass    : PASSWORD,
port    : '27017',
db      : 'test',
options : 'poolSize=10'
};

var configUrl = 'mongodb://'    +
              config.user + ':' + 
              config.pass + '@' + 
              config.host + ':' + 
              config.port + '/' +
              config.db;


var mongo   = mongoLite.connect(configUrl);
var server  = express.createServer();

server.get('/test/:id', function (req, res) {               
mongo.collection('test_' + req.params.id).insert({_id: count++}, function(err, doc){
    if (err)
        throw err;
});
res.end('inserted');
});

module.exports = server; 

server.js

var cluster = require('cluster');

cluster('./app')
    .use(cluster.reload())
    .use(cluster.logger('logs'))
    .use(cluster.stats())
    .use(cluster.pidfiles('pids'))
    .use(cluster.cli())
    .use(cluster.repl(8888))
    .listen(81);