@Entity
@Getter
@Setter
public class A {

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE,generator = "ID_GENERATOR")
  @SequenceGenerator(name="ID_GENERATOR",sequenceName = "a_id_seq")
  private Long id;

  private String name;

  @OneToMany(mappedBy = B_.A,cascade = CascadeType.ALL)
  List<B> bList;


}

@Entity
@Getter
@Setter
public class B {

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE)
  private Long id;

  private String name;

  private Long aId;

  @ManyToOne
  @JoinColumn(name=B_.A_ID,insertable = false,updatable = false)
  private A a;
}

@Service
public class BatchTestingService {

  private List<B> getTestListB() {
    List<B> bList = new ArrayList<>();
    for (int i = 0; i < 50; i++) {
      B b = new B();
      b.setName("b" + i);
      bList.add(b);

    }
    return bList;
  }

  public List<A>getTestListA(){
    List<A> aList = new ArrayList<>();
    for (int i = 0; i < 50; i++) {
      A a = new A();
      a.setName("a" + i);
      a.setBList(getTestListB());
      aList.add(a);

    }
    return aList;
  }

  }

 @Transactional
  @PostMapping
  public String saveAllTest(){
    var list=batchTestingService.getTestListA();
    aRepository.saveAll(list);
    return "OK";
  }

create sequence a_id_seq increment 50;
create sequence b_id_seq;
create table A(
  id bigint DEFAULT nextval('a_id_seq') primary key ,
  name text
);

create table B(
  id bigint DEFAULT nextval('b_id_seq') primary key,
  name text,
  a_id bigint references a on delete cascade

)

@Entity
@Getter
@Setter
public class A {

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE,generator = "ID_GENERATOR")
  @SequenceGenerator(name="ID_GENERATOR",sequenceName = "a_id_seq")
  private Long id;

  private String name;

  @OneToMany(mappedBy = B_.A,fetch = FetchType.LAZY,cascade = CascadeType.ALL)
  List<B> bList;
  
}

@Entity
@Getter
@Setter
public class B {

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "ID_GENERATOR")
  @SequenceGenerator(name = "ID_GENERATOR", sequenceName = "b_id_seq")
  private Long id;

  private String name;

  @ManyToOne
  @JoinColumn(name = "aId", nullable = false, updatable = false)
  private A a;
  
}

@Entity
@Getter
@Setter
public class A {

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "ID_GENERATOR")
  @SequenceGenerator(name = "ID_GENERATOR", sequenceName = "a_id_seq")
  private Long id;

  private String name;

  @OneToMany(targetEntity = B.class, fetch = FetchType.LAZY, cascade = CascadeType.ALL)
  @JoinColumn(name = "a_id", nullable = false)
  List<B> bList;

}

@Entity
@Getter
@Setter
public class B {

  @Id
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "ID_GENERATOR")
  @SequenceGenerator(name = "ID_GENERATOR", sequenceName = "b_id_seq")
  private Long id;

  private String name;
  
}

 1824200 nanoseconds spent preparing 54 JDBC statements;
    127689200 nanoseconds spent executing 51 JDBC statements;
    21552140300 nanoseconds spent executing 11 JDBC batches;