In this first installment of performance tuning tricks for ASP.NET and IIS 7 we will look at some of the easy, yet powerful possibilities in the web.config file. By taking advantage of these few tricks we can increase the performance of any new or existing website without changing anything but the web.config file.

The following XML snippets must be placed in the <system.webServer> section of the web.config.

HTTP compression

You’ve always been able to perform HTTP compression in ASP.NET by using third-party libraries or own custom built ones. With IIS 7 you can now throw that away and utilize the build-in compression available from the web.config. Add the following line to enable HTTP compression:

<urlCompression doDynamicCompression="true" doStaticCompression="true" dynamicCompressionBeforeCache="true"/>

By default, only text based content types are compressed.


Setting this attribute to true enables compression of dynamically generated content such as pages, views, handlers. There really aren’t any reasons not to enable this.


This attribute allows you to decide whether or not you want static files such as stylesheets and script files to be compressed. Images and other non-text content types will not be compressed by default. This is also something you want to enable.


If you do output caching from within your ASP.NET website, you can tell IIS 7 to compress the output before putting it into cache. Only if you do some custom output caching you might run into issues with setting this to true. Try it and test it. If your website works with this enabled, then you definitely want to keep it enabled.


By default, only text based content types are compressed. That means if you send application/x-javascript as content type, you should change it to text/javascript. If you use some custom modules in your website, then you might experience conflicts with the IIS 7 compression feature.


Cache static files

To speed up the load time for the visitors, it is crucial that everything that can be cached by the browser IS cached by the browser. That includes static files such as images, stylesheets and script files. By letting the browser cache all these files means it doesn’t need to request them again for the duration of the cache period. That saves you and your visitors a lot of bandwidth and makes the page load faster. A well primed browser cache also triggers the load and DOMContentLoaded event sooner.

By adding this snippet to your web.config, all static files are cached in the browser for 1 year:

     <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="365.00:00:00"/>

This setting sets the expiration date of the file one year in the future. It does that by setting an HTTP header that instruct the browser to add the file to its internal cache. If you hit F5 or ctrl-F5, the browser will request the files no matter what the expiration is set to.

A major problem with client-side caching is if your static files change before the cache expires. Then the visitor with the old version in the cache won’t see the new file until she clears the browser cache or hit F5. Therefore, this setting must be used with caution and probably with a shorter expiration time. In part 2 of this series I’ll address this problem and provide a simple solution to it.


Make sure that user sensitive information isn't cached on the browser. It will then be available by anyone else using the same browser.



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