I have used Nhibernate in my project at a company. But beyond simple queries we hit a performance issue. For a complex query, it took about 10 seconds to load the data to the frontend. So we had to rewrite the "code logic" ( the Nhibernate part ) into an "custom sql query" (Nhibernate allows you to write custom queries ) and then write custom mappers to map the results back to our application. The reason is Nhibernate does its processing objectwise and inmemory.