In VS2008, you still have to run the Wizard to create a TfsBuild.proj file for your build definition. That wizard can be really slow to open. In fact, internally the wizard can seem to hang indefinitely. And if you have a large number of files checked into your Team Project you may experience the same thing. But, there is something you can do.
When the wizard opens it queries the version control server for all SLN and VSMDI files in the Team Project. The larger the Team Project the longer this takes.
In VS2008 we limited this exhaustive search to the workspace that is defined in the build definition. So, if you limit the number of files in the workspace, the query will run much faster.
Because the definition dialog is where you define the workspace and launch the wizard, we don't force you to do one before the other. And the default workspace is the entire team project. That means that if a user tries to run the wizard first, they will run into the exact same slowness that was there in Version 1.
There is not a solution for this second problem except user training. Hopefully, you don't have too many people that are creating build definitions.