Sketch of a function to add artificial delays to XHRs

delayer.js

function wrapInDelay(obj, methodName, delay) {
    initialize();

    wrapMethod(obj, methodName, function (method, args) {
        window._delayerWantsDelay = delay;
        var result = method.apply(this, args);
        delete window._delayerWantsDelay;
        return result;
    });

    function initialize() {
        if (window._delayerInited)
            return;

        window._delayerInited = true;

        wrapMethod(jQuery, 'ajax', ajaxWrapper);
    }

    function ajaxWrapper(ajaxMethod, args) {
        if (window._delayerWantsDelay) {
            var delay = +window._delayerWantsDelay || 0;
            setTimeout(function () {
                ajaxMethod.apply(this, args);
            }, delay);
        }
        else {
            return ajaxMethod.apply(this, args); 
        }
    }

    function wrapMethod(obj, methodName, wrapper) {
        var original = obj[methodName];

        obj[methodName] = function () {
            var args = Array.prototype.slice.call(arguments, 0);
            return wrapper.call(obj, original, args);
        };
    }
}