WWW is also encoding the download into proper objects - are you accounting for the post-download initialization in your comparison?

I've had some success increasing WWW request performance by building the POST data myself rather than using the WWWForm object:

    private string generatePostString(){

    //MW: Generate a random salt for this request
    System.Random random = new System.Random();
    int randomNumber = random.Next(0, 9999999);


    //MW: hash the salt and secret key
    string salt = randomNumber.ToString();
    string hashedKey = Md5Sum(mSecretKey + salt);

    System.Text.StringBuilder sb = new System.Text.StringBuilder();

    sb.AppendLine("hashedKey="+ hashedKey +"&salt="+ salt +"&deviceType=5&bindingVersion=0.1");

    foreach (DictionaryEntry item in mParameters){
       //MW: TODO We need to distinguish between data types and correctly handle each one.
       sb.AppendLine("&params["+item.Key.ToString()+"]="+item.Value.ToString());
    }

    return sb.ToString();

}


private byte[] convertPostStringToData(string postString){

    //MW: convert the string into a byte array
    ASCIIEncoding encoding = new ASCIIEncoding();
    return encoding.GetBytes(postString);

}

Also, I suspect the additional time for a WWW request is because it has to load the crossdomain.xml for every request which is pretty annoying.