Before assuming it is relection. I would strongly recommend downloading (as a free trial at least) NHProf to see what is taking NHibernate so long. Or some other database profiler.
If I had to guess it was the number of database updates that is required here but I wouldn't guess about performance without getting some metrics first ;)
For instance it might be that you need to increase your batch size in NHibernate if you are doing a lot of small updates in one session, which can be done in your nh config file
<property name="adonet.batch_size">300</property>