If you are writing a WCF LOB adapter that can be consumed through BizTalk, you need to be aware of certain issues that can manifest because of the way BizTalk interacts with WCF adapters. Some of these can have performance and scalability impact and hence you should consider them when designing/configuring the adapter.

1. Processing in IConnection/IConnectionFactory instance – When using SSO, for each message, BizTalk will end up creating a new IConnectionFactory and a new IConnection instance. If your adapter is doing a lot of processing in either of those instances, it can cause significant performance impact. So you will need to think of alternatives – possibly doing the processing upfront and caching.
2. Caching of LOB artifacts on IConnection instance – Typically you will have some LOB artifacts that will comprise the context associated with an IConnection instance. If your adapter is using WCF LOB Adapter SDK’s connection pooling, in the non SSO scenario, the IConnection instances will be closed only when the idle connection timeout expires. So you need to think about freeing up the LOB resources that comprise your connection context. If you make idle connection timeout too small, it would impact performance since LOB connection creation will incur an overhead and if you keep it at a large value then you will be holding on to LOB artifacts for a longer time than you really need to, possible scalability/performance impact.
3. Throughput stall – You can find more details about the issue on http://blogs.msdn.com/adapters/archive/2008/08/18/throughput-stalls-when-using-adapters-developed-against-the-wcf-lob-adapter-sdk-in-biztalk.aspx.  Note that if you don’t limit the number of concurrent connections through your adapter, you won’t run into this problem. The problem happens the contention between Open and Send/Execute for thread pool threads.
4. Writing/promoting properties to BizTalk message context – When the adapter receives a WCF message from BizTalk, it will have all the properties comprising the BizTalk message context. For messages that originate from the adapter, if you want to have properties either written/promoted to BizTalk message context, you can look at http://technet.microsoft.com/en-us/library/bb246105.aspx on how to go about doing that.  
5. Transaction overhead – BizTalk will by default start a transaction and send the message/process the response in that transaction scope. This can cause a significant overhead particularly if say the LOB doesn’t support transactions. In BizTalk 2009, there is an option on the Send port that lets you configure whether transactions should be enabled or not.