I forgot to mention that my mapping files are XML (.hbm.xml).
I used Hibernate Tools for eclipse to create them. So, the format for a property is like this one:

<property name="web" type="boolean">
     <column name="web" not-null="true" />
</property>

I don't know is this old format generates more work to the mapping binder, but I don't think that this could cause such overload at startup.