ASP.NET monitoring page_load performance (without code change)

This isn't a WebForms concern, it's an ASP.net concern. Using either global.asax, or an IHttpModule hook into the BeginRequest/EndRequest events to track how long a request takes.

StopWatchModule.cs

class StopWatchModule: IHttpModule
{
    private Int64 _requestStartTicks; 

    public void Init(HttpApplication application)
    {
         application.Context.BeginRequest += Application_BeginRequest;
         application.Context.EndRequest += Application_EndRequest;
    }

    private void Application_BeginRequest(object sender, EventArgs e)
    {
        _requestStartTicks = Stopwatch.GetTimestamp();
    }

    private void Application_EndRequest(object sender, EventArgs e)
    {
        Int64 requestStopTicks = Stopwatch.GetTimestamp();

        Double requestDurationMs = (Double)(requestStopTicks - _requestStartTicks) / Stopwatch.Frequency / 1000000f; 
        /* 
           Sample output:
           Request for "~/EditCustomer.aspx" took 37 ms to complete
        */
        HttpContext context = ((HttpApplication)sender).Context;
        Logger.Info("Request for \"{0}\" to {1} ms to complete", 
              context.Request.AppRelativeCurrentExecutionFilePath, 
              requestDurationMs.ToString("r", CultureInfo.InvariantCulture));
    }
}

Web.config

<configuration>
    <system.web>
        <httpModules>
            <add name="StopWatchModule" type="StopWatchModule" />
        </httpModules>
    </system.web>
</configuration>

I'm writing from memory so there may be errors above, but that's the idea. your log message can be as detailed as you want. and Logger is some type of logging library. I prefer log4net, but the choice is yours.