Problém s výkonem při aktualizaci tabulky z jiné tabulky

Zkuste použít merge.

merge into old_customer_source t1
using (select t2.custid, t2.firstname, t2.lastname
         from  new_customer_source t2
      ) t2
 on (t1.custid = t2.custid)
when matched then
update set t1.firstname = t2.firstname, 
           t1.lastname = t2.lastname
;