From MSDN:

Welcome to the first edition of the Microsoft® BizTalk® Server Performance Optimizations Guide. We created this guide to provide in depth information for optimizing the performance of a BizTalk Server solution. Full end-to-end performance testing is frequently overlooked during enterprise application deployment. Knowing that Microsoft has built a scalable messaging infrastructure, many organizations that use BizTalk Server spend little or no time conducting performance testing of their own applications. BizTalk Server applications consist of many parts, which may include custom-built components as well as those provided by Microsoft. It is impossible for Microsoft to performance test every possible combination of these components. Therefore, fully and properly conducting a performance test of your application is a critical step of any deployment. The purpose of this guide is to consolidate and provide prescriptive guidance on the best practices and techniques that should be followed to optimize BizTalk Server performance.

Read more about it or get it.