Output caching is one of the major factor to load your page more faster. up to now ASP.NET allow developer to store generated output of page, control and HTTP request into memory to used further request.
ASP.NET 4.0 provides the mechanism to let developer handle output cache by creating your own custom output-cache provider and manage the persistency of HTML content. For doing this you just need to create class which derives from System.Web.Caching.OutputCacheProvider type and add it to your caching section of web.config. The default provider is AspNetInternalProvider.
In addition, you can choose different provider for different pages! There are two way to do this.
First you need to override one new method in Global.asax called GetOutputCacheProviderName which helps you to select output cache provider.
Adding code at request level is like adding more work to do, the easiest way is to select output-cache provider is to set value of attribute providerName which is part of OutputCache page or control directive.
Topic: ASP.NET 4.0 and Visual Studio 2010 Web Development Beta 2 Overview