With BTS2006 in full force, the BTS team is frequently fielding performance related questions. There is a lot of information already available for customers to reference; the product documentation is a great place to start.
The Performance and Capacity section in the core docs covers a wide range of topics. The 64-Bit Support article delves into components of BizTalk that are supported in 64 bit and also answers some basic FAQ regarding this topic.
There is a section that covers planning for sustained performance. It contains articles that provide useful guidance on how to include performance considerations during the design, implementation and release phases of BTS2K6 design. It also highlights how to set realistic performance goals and how to measure them. The documentation also contains guidance on scaling solutions as well as tips and tricks to improve performance. In addition to this, there is a paper available on how to manage a successful performance lab here.
Once a solution is deployed, there are also articles in msdn that cover the performance counters available in BizTalk to detect bottlenecks.
And if you just want to look at hard numbers, a comparative adapter study compares the performance of BizTalk Server 2004 against BizTalk Server 2006 adapters.