Performance Testing

Updated on Nov 6, 2025

To test application performance, add rules using FiddlerScript to the OnBeforeResponse function (except where noted). For example:

Simulate modem uploads (add to OnBeforeRequest function)

// Delay sends by 300ms per KB uploaded.
oSession["request-trickle-delay"] = "300";

Simulate modem downloads

// Delay receives by 150ms per KB downloaded.
oSession["response-trickle-delay"] = "150";

Flag content which isn't set to cache on the client.

if (!(oSession.oResponse.headers.Exists("Expires") 
| | (oSession.oResponse.headers.ExistsAndContains("Cache-Control", "age")))
| | (oSession.oResponse.headers.Exists("Vary"))){
{
oSession["ui-color"]="brown"; // Use C# color strings here.
oSession["ui-italic"]="true"; 
}

Display in the "Custom Column" the number of milliseconds from the moment of the request until the last byte was received.

oSession["ui-customcolumn"] = oSession["X-TTLB"];

Display the # of milliseconds until the First Byte was received from the server, followed by the # of ms until the Last Byte.

oSession["ui-customcolumn"] = "FB: " + oSession["X-TTFB"] + "; LB: " + oSession["X-TTLB"];

Add a CopyTimers context menu item to the Session List (Scope is Global)

public static ContextAction("CopyTimers")
function CopyTimers(oSessions: Fiddler.Session[]){
	if (null == oSessions){
	MessageBox.Show("Please select sessions to copy timers for.", "Nothing to Do");
	return;
	}

	var s: System.Text.StringBuilder = new System.Text.StringBuilder();

	for (var x = 0; x < oSessions.Length; x++)  {
	s.AppendFormat("{0}, {1}, {2}, {3}, {4}, {5}, {6}, {7}\r\n",
	oSessions[x].Timers.ClientConnected,
	oSessions[x].Timers.ClientDoneRequest,
	oSessions[x].Timers.ServerConnected,
	oSessions[x].Timers.ServerGotRequest,
	oSessions[x].Timers.ServerBeginResponse,
	oSessions[x].Timers.ServerDoneResponse,
	oSessions[x].Timers.ClientBeginResponse,
	oSessions[x].Timers.ClientDoneResponse
	);
	}
	Utilities.CopyToClipboard(s.ToString());
	MessageBox.Show("Done.");
}