I'm comparing it Java where you can start your application server in debug mode, then attach your IDE to the server. And you can change your code "on the fly" without restarting the server. As long as your changes don't affect any method signatures or fields you can just hit recompile for a class and the application server (servlet container) will reload the class.
I suppose this is impossible in ASP.NET since all classes are packed into assemblies and you cannot unload/reload assemblies, can you ?
So when you have an .aspx page and an assembly deployed to GAC and your codebehind changes you have to redeploy the assembly and reset IIS. I'm talking about Sharepoint applications in particular and I'm not sure whether you have to do iisreset for private assemblies but I guess you have too.
So the best way to debug aspx pages with code behind I guess would be to get rid of the codebehind for the time of active debugging and move into the page, then when it is more or less working move it back to codebehind. (This would be applicable only for application pages in Sharepoint, site pages don't allow inline code )
How do you approach debugging of your ASP.NET applications to make it less time consuming?