This warning is raised when the profiler detects that you are writing a lot of data to the database. Similar to the warning about too many calls to the database, the main issue here is the number of remote calls and the time they take.
We can batch together several queries using NHibernate's support for MultiQuery and MultiCriteria, but a relatively unknown feature for NHibernate is the ability to batch a set of write statements into a single database call.
This is controlled using the adonet.batch_size setting in the configuration. If you set it to a number larger than zero, you can immediately start benefiting from reduced number of database calls. You can even set this value at runtime, using session.SetBatchSize().