After doing some load testing we found that the threading was different from the custom membership provider and was clashing with the normal way of getting the user from the data context being shared by the business object. So what we did is to create a different method for get user in the custom membership provider that creates its own context with a using statement. This has had a big improvement on the application and was something that didn't really show up until the application came under load.

So what we learned from load testing was to always use ToList when returning an IEnumerable from the data project so that the context is not required for later manipulations, disable pooling, and give requests for data from threads outside the normal application their own context. These three things have really helped the application work much better.