I would like to use nHibernate into a .NET project i have
but people that are more experience in .NET than me told me
not to do it because of the reflection the nHibernate use. They say that reflection in the .NET is way too slow and nHibernate will kill the performance of the application. Is this true ?

NHibernate uses reflection when the SessionFactory is created, which usually is when the applications starts. Code for setting the values in your objects is also generated at startup, with help from reflection, so reflection isnt used when you do CRUD-operations.

I'm not too familiar with the core of NHibernate, so I wont go into details. I will let someone else do that. :)

Yes, NHibernate uses reflection, but the impact of reflection is often exagerrated. Often, a single database call will impact performance much more than thousands of reflection calls.