I am writing an upload speed test in Javascript. I am using Jquery (and Ajax) to send chunks of data to a server in order to time how long it takes to get a response. This should, in theory give an estimation, of the upload speed.

Of course to cater for different bandwidths of the user I sequentially upload larger and larger amounts of junk data until a threshold duration is reached. Currently I generate the junk data using the following function, however, it is very slow when generation megabytes of data.

function generate_random_data(size){
    var chars = "abcdefghijklmnopqrstuvwxyz";
    var random_data = "";
    for (var i = 0; i < size; i++){
        var random_num = Math.floor(Math.random() * char.length);
        random_data = random_data + chars.substring(random_num,random_num+1);
    }
    return random_data;

Really all I am doing is generating a chunk of bytes to send to the server, however, this is the only way I could find out how in Javascript.

Any help would be appreciated.