@kettch:  Si Senor.  The CustomRules.js by default has

        if (m_SimulateModem){
            // Delay sends by 300ms per KB uploaded.
            oSession["request-trickle-delay"] = "300"; 
            // Delay receives by 150ms per KB downloaded.
            oSession["response-trickle-delay"] = "150"; 

So it seems to apply a delay based on the volume of data.  That seems reasonable for throttling bandwidth.

Plus this Fiddler Extension http://fiddlerdelayext.codeplex.com/ for latency looks like a winner combination.