Serialization with relationships very slow with Marshmallow

Question

I have the following example table in my DB which I manage mit Flask_SQLAlchemy.

class Employee(db.Model):
    __tablename__ = 'employee'
    id = db.Column(db.Integer, primary_key=True)
    salutation = db.Column(db.String(250), nullable=False)
    first_name = db.Column(db.String(250), nullable=False)
    last_name = db.Column(db.String(250), nullable=False)
    
    employments = db.relationship('Employment', foreign_keys='Employment.employee_id', backref="employee", lazy=True)

my Schema for Flask looks like this:

class EmployeeSchema(mm.SQLAlchemyAutoSchema):
    class Meta:
        model = DB_employee.Employee
        include_fk = True
        include_relationships = True

The following is my routing for GET of the employees:

class EmployeesListAPI(Resource):
    def __init__(self):
        self.reqparse_get = reqparse.RequestParser()
        self.reqparse_get.add_argument('employee_id', type=int, action='append', required=False, location='args')
        self.reqparse_get.add_argument('salutation', type=str, action='append', required=False, location='args')
        self.reqparse_get.add_argument('first_name', type=str, required=False, location='args')
        self.reqparse_get.add_argument('last_name', type=str, required=False, location='args')
        
        super(EmployeesListAPI, self).__init__()

    def get(self):
        get_args = self.reqparse_get.parse_args()
        employees = get_list_employee(get_args)
        
        employees_output = EmployeeSchema(many=True, only=get_args['return_fields']).dump(employees)
            
        return employees_output, 200

if I now run this, the query with SQL-Alhchemy takes about 0.01 seconds which is ok for me, but the serialization with Flask_Marshmallow takes about 1.3 seconds.

if I change the Schema as following: include_relationships = False and run the serialization manually:

for employee, output in zip(employees, employees_output):
            if get_args['show_employments']:
                output['employments'] = [e.id for e in employee.employments]

the serialization with marshmallow takes about 0.02 seconds and the manually added employments-relationships takes about 1.2 seconds.

In my real case, I have much more then just one relationship an the serialization of the data takes an astonishing 32 seconds.

is there a way to speed up the serialization with marshmallow? Am I doing something wrong?

Thank you for your support.