If you are using the VS 2005 Web Application project option, here are a few optimizations you might want to consider:
1) If you have a very large project, or are working on an application with many other developers, you might want to consider splitting it up into multiple "sub-web" projects. I wouldn't necessarily recommend this for performance reasons (unless you have thousands and thousands of pages it probably doesn't make a huge difference), but it can sometimes make it easier to help manage a large project. Please read this past blog-post of mine on creating sub-web projects to learn how to use this.
2) Consider adding a VS 2005 Web Deployment project to your solution for deep verification. I mentioned above that one downside of using the VS 2005 Web Application Project option was that it only compiled the code-behind source code of your pages, and didn't do a deeper verification of the actual .aspx markup (so it will miss cases where you have a mis-typed tag in your .aspx markup). This provides the same level of verification support that VS 2003 provided (so you aren't loosing anything from that), but not as deep as the Web Site Project option. One way you can still get this level of verification with VS 2005 Web Application Projects is to optionally add a VS 2005 Web Deployment Project into your solution (web deployment projects work with both web-site and web-application solutions). You can configure this to run only when building "release" or "staging" builds of your solution (to avoid taking a build hit at development time), and use it to provide a deep verification of both your content and source code prior to shipping your app.
Common Tips/Tricks for Optimizing any VS 2005 Build Time
Here are a few things I recommend checking anytime you have poor performance when building projects/solutions (note: this list will continue to grow as I hear new ones - so check back in the future):
1) Watch out for Virus Checkers, Spy-Bots, and Search/Indexing Tools
VS hits the file-system a lot, and obviously needs to reparse any file within a project that has changed the next time it compiles. One issue I've seen reported several times are cases where virus scanners, spy-bot detecters, and/or desktop search indexing tools end up monitoring a directory containing a project a little too closely, and continually change the timestamps of these files (they don't alter the contents of the file - but they do change a last touched timestamp that VS also uses). This then causes a pattern of: you make a change, rebuild, and then in the background the virus/search tool goes in and re-searches/re-checks the file and marks it as altered - which then causes VS to have to re-build it again. Check for this if you are seeing build performance issues, and consider disabling the directories you are working on from being scanned by other programs. I've also seen reports of certain Spybot utilities causing extreme slowness with VS debugging - so you might want to verify that you aren't having issues with those either.
2) Turn off AutoToolboxPopulate in the Windows Forms Designer Options
There is an option in VS 2005 that will cause VS to automatically populate the toolbox with any controls you compile as part of your solution. This is a useful feature when developing controls since it updates them when you build, but I've seen a few reports from people who find that it can cause VS to end up taking a long time (almost like a hang) in some circumstances. Note that this applies both to Windows Forms and Web Projects. To disable this option, select the Tools->Options menu item, and then unselect the Windows Forms Designer/General/AutoToolboxPopulate checkbox option (for a thread on this see: http://forums.asp.net/1108115/ShowPost.aspx).
3) Examine which 3rd party packages are running in Visual Studio
There are a lot of great 3rd party VS packages that you can plug into Visual Studio. These deliver big productivity wins, and offer tons of features. Occasionally I've seen issues where performance or stability is being affected by them though. This is often true in cases where an older version (or beta) of one of these packages is being used (always keep an eye out for when a manufacturer updates them with bug-fixes). If you are seeing issues with performance or stability, you might want to look at trying a VS configuration where you uninstall any additional packages to see if this makes a difference. If so, you can work with the 3rd party manufacturer to identify the issue.
Visual Basic Build Performance HotFix
The Visual Basic team has released several hotfixes for compilation performance issues with large VB projects. You can learn how to obtain these hotfixes immediately from this blog post. The VB team also has a direct email address -- firstname.lastname@example.org -- that you can use to contact them directly if you are running into performance issues.
Hope this helps,