Often ignored, but a really important, persistence points in Biztalk are the key issue in dealing with Orchestration Performance. Many applicative problem come from a misunderstanding of this concept.
Sandro perreira (a great biztalk MVP) wrote a great article about it : http://sandroaspbiztalkblog.wordpress.com/2009/10/23/biztalk-orchestration-%E2%80%93-understanding-persistence-points/
You should read it,
All BizTalk developper must be careful to those concepts and organize their developpement around it.
A common mistake is the send port in a while shape, more than 50 send don’t forget to use atomic scope.
Another one, be careful with custom c# object and the serialization way.