ASP.NET Development Server or Localhost IIS?

Another distinction I noticed is that Cassini runs as a 32-bit process and you have no control over it, whereas you can control the application pool of your IIS app to disallow 32-bit (assuming your IIS is running on a 64-bit server). This becomes especially important if your web application is going to call APIs in 64-bit processes such as SharePoint Foundation/Server 2010. When you debug your web app with Cassini as your debug server, you'll get "The Web application at url could not be found. Verify that you have typed the URL correctly" type errors when instantiating objects. If you debug using IIS with the app running in an app pool that runs as 64-bit with an identity that allows access to sharepoint database then you'll be able to debug properly.

In VS12 the development server is way slow, takes a few seconds to download a 2kbyte file. This did not happen in vs10. When you have a bunch of jquery files and css this is a real problem. Also every page requeries all the css/js files. Very very slow regression testing.