I have below block, and it is taking 8 to 9 hours for 450k data to complete. How to improve performance in that case? tab_a is having 10M data.

Declare

cursor cur_dup is
Select col_a,col_b,count(1) from tab_a group by col_a,col_b having count(1) > 1;
Begin
  for cur_dup_rec in cur_dup loop
     
    update tab_a
    set qty = 0
    where col_a = cur_dup_rec.col_a and col_b = cur_dup_rec.col_b
    and trunc(u_created_date) <> (Select max(trunc(u_created_date)) from tab_a 
                                   where col_a = cur_dup_rec.col_a and col_b = cur_dup_rec.col_b);
                                   
    update tab_a
       set (u_modified_date) = trunc(sysdate)
     where col_a = cur_dup_rec.col_a and col_b = cur_dup_rec.col_b;
     
     for cur_dup2_rec in (Select distinct item,customer,location,requested_date 
                            from tab_a 
                           where col_a = cur_dup_rec.col_a and col_b = cur_dup_rec.col_b) loop
                           
           update tab_a
            set (u_modified_date) = trunc(sysdate)
            where item = cur_dup2_rec.item and customer = cur_dup2_rec.customer and location = cur_dup2_rec.location
            and to_char(requested_date,'DD-MON-YYYY') = cur_dup2_rec.requested_date;
            
            
     end loop;
  
  end loop;
End;