If we look inside documentation, we can see this:
There is a general performance consideration with Class Table Inheritance: If the target-entity of a many-to-one or one-to-one association is a CTI entity, it is preferable for performance reasons that it be a leaf entity in the inheritance hierarchy, (ie. have no subclasses). Otherwise Doctrine CANNOT create proxy instances of this entity and will ALWAYS load the entity eagerly.
I think it can be improved, if we will load only discriminator column value for resolve target class name, instead of loading whole entity. When we perform query from root entity, dicriminator value is already present in fetched database row.
What do you think?