Performance testing

  • Timeline for performance analysis

    Fiddler’s Timeline shows the start time and duration of each selected session – get to know at a glance how your app is using the network. You can view between 1 and 250 selected sessions as a “waterfall” diagram, which is useful both for performance analysis and to understand how requests relate to one another. Easily identify slow requests (long bars), bottlenecks where requests are blocked due to connection limits (stair-step groups of requests), and cases where connections were unnecessarily closed (red X icons). Using this information, you may be able to adjust your application to better order requests to optimize network performance.
  • Simulate HTTP compression

    You can use Fiddler to mimic HTTP compression. The Transformer Inspector allows you to add or remove HTTP-based encodings from the response. Find the selected session body’s size listed at the top of the Transformer tab – you can see this number changing as you add or remove encodings. This way you can determine how effective HTTP compression will be when applied to your content—for most textual types, compression reduces the body’s size by a total of 80%.
  • Flagging performance bottlenecks

    You can use Fiddler's Custom Rules to draw attention to potential performance or security problems. For instance, you can flag any response larger than 25KB. Similarly, you can mark responses that do not specify caching information. Use colors or various font styles to highlight the traffic that meets/doesn't meet characteristics you specified through the Rules menu item in advance.
  • Taking advantage of HTTP caching

    It’s common knowledge that the two key factors for improving the speed and performance of your web applications are:

    • Reducing the number of request/response roundtrips.
    • Reducing the number of bytes transferred between the server and the client.

    HTTP caching is one of the best ways to reduce roundtrips and bytes transferred. Caching provides a mechanism for a client or proxy to store HTTP responses for later use, so that requests don’t need to cross the network.


    An HTTP/304 response crosses the network much more quickly than if the full resource had been re-downloaded since it contains only headers and no body. However, even an HTTP/304 requires a full roundtrip to the web server; by carefully setting response headers, a web application developer can eliminate the need to issue even conditional requests.


    Fiddler exposes the Cache-Control and Expires headers for you to quickly evaluate if your application is receiving and sending back to the web server the appropriate caching information, this way guaranteeing optimum cache behavior and minimum transactions between the server and your application.


    What’s more, you can choose to compress certain HTTP responses in order to further limit the amount of bytes being transferred to the server.

  • Profiling the performance of your web app

    Check out Fiddler’s Statistics tab to get an idea of your app’s overall performance metrics. You can select all sessions to see the total number of requests and bytes sent and received, broken down by content type or in a pie chart. By exposing all HTTP(s) traffic, Fiddler easily shows which files are used to generate a given page: users can multi-select the number of requests and bytes transferred to get a "total page weight".


    The best way to impress your web site’s first time visitors is to deliver fewer and smaller files. A couple of tips for achieving better app performance:

    • Use fewer graphics
    • Extract styles into a single CSS file
    • Extract script blocks into a single JavaScript file
    • Simplify your page layout
    • Use HTTP Compression

    Once you've tuned your site for a fast first visit, you can make it even faster for returning users by taking advantage of HTTP caching.

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