For most developers XSLT is a black-box operation. Unlike code, which you can step through or profile, and SQL, which has query plans, all the processing for an XSLT transformation is hidden in a single method call.
Microsoft is opening the lid on that box for .NET developers by releasing their XSLT Profiler Addin for VS 2008. Even though it is just a Community Tech Preview, the profiler is only available for developers using a VS 2008 Team System SKU with the Performance Tools installed.
A quick spin shows that it is reasonably easy to use and is quite consistent with the other profiling tools. The Call Tree view gives a hierarchical view of how the templates were applied much like a call stack in imperative code. The Function view breaks down operations, listing both how much time they took solely and how long they took with child calls.
The documentation reflects the projects status as a preview. While well written, it does mention that at least one option is controlled by directly editing the Windows registry.