Jpa @CollectionTable very slow query

I have this entity

@Entity
@Table(name = "GEO_MUNICIPALITY", indexes = {
        @Index(name = "idx_municipalityentity", columnList = "PROVINCE_CODE")
})
...
public class MunicipalityEntity {
    @Id
    @Column(name = "MUNICIPALITY_CODE", nullable = false)
    private String code;

    @Column(nullable = false)
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "PROVINCE_CODE")
    private ProvinceEntity province;

    @ElementCollection
    @CollectionTable(name = "GEO_MUNICIPALITY_POSTAL_CODE",
            joinColumns = @JoinColumn(name = "MUNICIPALITY_CODE"),
    indexes = {
            @Index(name = "idx_municipalityentity_postal_code", columnList = "MUNICIPALITY_CODE")
    })
    @Column(name = "POSTAL_CODE")
    private List<String> postalCodeList;
}

@Entity
@Table(name = "GEO_PROVINCE")
...
public class ProvinceEntity {
    @Id
    @Column(nullable = false)
    private String code;
...
}

Now my apis need to display all municipalities (with all municipality postal codes) with a given province code, so i have a repository class like this:

@Repository
public interface MunicipalityJpaRepository extends JpaRepository<MunicipalityEntity, String> {
    List<MunicipalityEntity> findAllByProvinceCode(String province);

    @Query("select m from MunicipalityEntity m left join m.postalCodeList postalCodeList where m.province.code = ?1")
    List<MunicipalityEntity> findByCode(String code);
}

The two methods do the same thing and I tried in both way (an the result is the same) because when i call getPostalCodeList() the query is very slow because it make a select for every result of municipality (to retrive the postal codes).

How can I improve the performances? Also, since I always search municipalities by code, is a good idea to make the index like in the code? Thank you!

I tried both using jpa repository and custom query. I was expecting a fast query execution time but the execution is very slow because after the join query the application make a query for each resulted municipality, to fetch the postal codes.