Same SQL Query Slower from NHibernate Application than SQL Studio?

Our application issues an NHibernate-generated SQL query. At application runtime, the query takes about 12 seconds to run against a SQL Server database. SQL Profiler shows over 500,000 reads.

However, if I capture the exact query text using SQL Profiler, and run it again from SQL Studio, it takes 5 seconds and shows less than 4,600 reads.

The query uses a couple of parameters whose values are supplied at the end of the SQL text, and I'd read a little about parameter sniffing and inefficient query plans, but I had thought that related to stored procedures. Maybe NHibernate holds the resultset open while it instantiates its entities, which could explain the longer duration, but what could explain the extra 494,000 "reads" for the same query as performed by NHibernate? (No additional queries appear in the SQL Profiler trace.)

The query is specified as a LINQ query using NHibernate 3.1's LINQ facility. I didn't include the query itself because it seems like a basic question of philosophy: what could explain such a dramatic difference?

In case it's pertinent, there also happens to be a varbinary(max) column in the results, but in our situation it always contains null.

Any insight is much appreciated!