Simple measure of round-trip times for bouncing a JSON request off a server.  The server-side script (bounce.php) is:

<?php
$a = array(
    'date'  => date('Y.m.d H:i:s'),
    'uname' => php_uname()
);
echo json_encode($a);
?>

The code to generate a JSON request (in bounce.html) is in effect:

var o = JSON.makeGET("bounce.php");
o.onResponse = function(v) {
    // use v
};
o.sendGET();

Where v is the JSON response (click bounce.php to see a response).