this script took 48 hrs to complete 500k records

this script snippet cause the script to take 48+ hours to complete. The update is only 500K records.

the date data that is being compared is loaded from a prior script. So, I know the date format is correct. Similar scripts like this run in 10 minutes.

What is wrong in this code??
-----------code starts------------------------------------------

begin
   open data_cursor;
   loop
   fetch data_cursor into data_val;
   exit when data_cursor%NOTFOUND;
        update NSF_account_HOUSEHOLD set
      hh_contribution_income = data_val.contribution_income,
      hh_balance = data_val.balance,
      hh_nsf_fees = data_val.nsf_fees,
      hh_nsf_waive_bank_error = data_val.nsf_waive_bank_error,
      hh_nsf_waive_cap = data_val.nsf_waive_cap,
      hh_nsf_waive_other = data_val.nsf_waive_other,
      hh_return = decode(sign(data_val.balance - 100), -1, 0, data_val.contribution_income * 2 /
         data_val.balance) * 100,
      hh_referral_code = decode(data_val.acct_officer_num, 0, 'R', 'O') ||
         decode(sign(months_between(data_val.data_date,data_val.open_date) -5), -1, 'N',
         decode(decode(sign(ceil((decode(sign(NSF_account_HOUSEHOLD.acct_balance-100), -1, 1.5,
         .015 * data_val.balance) - data_val.contribution_income * 2) / 35)), -1, 0,
         ceil((decode(sign(data_val.balance - 100), -1, 1.5, .015 * data_val.balance)
         - data_val.contribution_income * 2) / 35)),0,'0','1')),
      hh_minimum_nsfs = decode(sign(ceil((decode(sign(NSF_account_HOUSEHOLD.acct_balance-100), -1, 9999,
         .015 * data_val.balance) - data_val.contribution_income * 2) / 35)), -1, 0,
         ceil((decode(sign(data_val.balance - 100), -1, 1.5, .015 * data_val.balance)
         - data_val.contribution_income * 2) / 35)),
      hh_first_deposit_open_date = data_val.open_date,
      hh_deposit_tenure=months_between(data_val.data_date,data_val.open_date),
      hh_best_chk_minimum_nsfs=data_val.best_nsfs  
      where NSF_account_HOUSEHOLD.data_date = data_val.data_date
      and NSF_account_HOUSEHOLD.HOUSEHOLD_KEY = data_val.HOUSEHOLD_KEY;
    end loop;

end;
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