There are the main sources of the information of the subj:

The blog of the BizTalk Performance
The articles in the MSDN:
“BizTalk Server 2006: Managing a Successful Performance Lab ”
“BizTalk Server 2004: Performance Tuning for Low Latency Messaging”
The BizTalk Help articles:
Performance and Capacity Planning”
Identifying Bottlenecks in the BizTalk Tier” (the list of the BizTalk System Performance Counters)