We discovered this same problem with nHibernate about 6 months ago. It was running index scans instead of seeks. The datbase columns were defined ad VARCHR but Hibernate was passing NVARCHAR. After we brought it to the attention of the people who developed the offending application they were able to make some modifications to the configuration parameters for nHibernate and corrected the problem. I am not familier enough with nHibernate to know exactly what they did.