Slow deletion of rows in table referenced by foreign key

create table tq84_parent (
    id integer,
    payload varchar2(2000),
   --
    constraint tq84_pk primary key (id)
);
 
create table tq84_child (
   pid integer not null,
   payload varchar2(2000),
   --
   constraint tq84_fk foreign key (pid) references tq84_parent
);
 
 
begin
 
   insert into tq84_parent
   select
      level,
      lpad('x', 2000, 'x')
   from
      dual connect by level <= 10000;
 
   insert into tq84_child
   select
      level,
      lpad('x', 2000, 'x')
   from
      dual connect by level <= 10000;
     
   commit;
end;
/
 
-- 0.2
 
 
delete from tq84_child;
-- 0.1
 
commit;
 
delete from tq84_parent;
-- 11.3
 
rollback;

create index tq84_child_ix on tq84_child(pid);
 
delete from tq84_parent;
-- 0.2

rollback;

drop index tq84_child_ix;
alter table tq84_child disable constraint tq84_fk;

delete from tq84_parent;

rollback;

drop table tq84_child;
drop table tq84_parent;