Microsoft has published a few days ago a new guide around optimization of BizTalk Server performance. This is a lengthy guide (over 200 pages), but it has a lot of very interesting information, some of which you can't find anywhere else, and there's always something to learn. Highly recommended. You can download it here.
A second recommendation is a little tool that is very helpful in gathering information about a BizTalk installation, and which also gives you a report about your installation. A little like the BizTalk Best Practices Analyzer, only deeper and with more information. It's MsgBoxViewer, and you can download version 9.20.3 here.