I encountered funny performance problem with IQuery.List():
(I hope that class names are self-explaining)

This query takes 9 seconds of *nhibernate time* i.e. to materialize objects:

from ProjectSubject ps inner join fetch ps.Subject s left join fetch s.Addresses a left join fetch a.Province p left join fetch a.County c left join fetch a.City city where ps.Project.ID = :projectId and coalesce(a.Flag, 1) = 1

This one takes less than 0,1 seconds and does luckily the same:

select ps, s, a from ProjectSubject ps inner join fetch ps.Subject s left join fetch s.Addresses a left join fetch a.Province p left join fetch a.County c left join fetch a.City city where ps.Project.ID = :projectId and coalesce(a.Flag, 1) = 1

regional units (Province, County and City) have one-to-many relationship with Address and latter has inverse many-to-one relationship with all three ones. Also, city has many-to-one relationship with county and county has same with province.

Can you isolate this behavior as much as possible and send the TestCase?

You may try it in another environment to be sure that it is not due to your installation...