Quick summary : we are introducing a new optimizeCompilations switch in ASP.NET that can greatly improve the compilation speed in some scenarios. There are some catches, so read on for more details. This switch is currently available as a QFE for 3.5SP1 , and will be part of VS 2010. To turn on this switch, add it to your compilation section: < compilation optimizeCompilations = " true " > What prompted us to add this switch The ASP.NET compilation system takes a very conservative approach which causes it to wipe out any previous work that it has done any time a ‘top level’ file changes. ‘Top level’ files include anything in bin and App_Code, as well as global.asax. While this works fine for small apps...