我建议使用一个包含查询的merge语句，返回准备好的不匹配记录进行更新。

merge into cust_info dst
using (
select l.row_id, l.cust_privilege_number
from legacy_cust_info lci
inner join cust_info ci on ci.row_id = lci.row_id
where ci.cust_privilege_number != lci.cust_privilege_number
) src
on (dst.row_id = src.row_id)
when matched then 
  update set dst.cust_privilege_number = src.cust_privilege_number;

还可以考虑为表创建索引：

CREATE INDEX legacy_cust_info#rid#cpn#idx ON LEGACY_CUST_INFO (row_id, cust_privilege_number);
CREATE INDEX cust_info#row_id#idx ON CUST_INFO (row_id);

UPD。您可以通过运行嵌套的select而不将其内容合并到目标表中来估计时间：

select count(1) from (
  select l.row_id, l.cust_privilege_number
   from legacy_cust_info lci
   inner join cust_info ci on ci.row_id = lci.row_id
   where ci.cust_privilege_number != lci.cust_privilege_number
)

UPD2.要考虑ci.cust_privilege_number中的NULL，只需在查询中添加替代条件：

where (
  ci.cust_privilege_number != lci.cust_privilege_number 
  or (
   ci.cust_privilege_number is null 
   and lci.cust_privilege_number is not null
  )
)

索引有助于更快地检索数据。检查以下查询：-
对于row_ID

CREATE INDEX idx_legacy_cust_info_row_id ON LEGACY_CUST_INFO (ROW_ID);
CREATE INDEX idx_cust_info_row_id ON CUST_INFO (ROW_ID);

对于单个update语句，使用两个表之间的联接来更新不匹配的CUST_PRIVILEGE_NUMBER。检查以下查询：-

UPDATE CUST_INFO CI
SET CI.CUST_PRIVILEGE_NUMBER = (
   SELECT LCI.CUST_PRIVILEGE_NUMBER
   FROM LEGACY_CUST_INFO LCI
   WHERE LCI.ROW_ID = CI.ROW_ID
)
WHERE CI.CUST_PRIVILEGE_NUMBER <> (
   SELECT LCI.CUST_PRIVILEGE_NUMBER
   FROM LEGACY_CUST_INFO LCI
   WHERE LCI.ROW_ID = CI.ROW_ID
);