RLS changes the text of the query, so you are potentially running a brand new query, which is then subject to the same standard SQL tuning requirements as normal SQL, ie, good statistics, right indexes, etc etc...
I would look at running:
ALTER SESSION SET events '10730 trace name context forever, level 1';
and then running your SQL with the RLS policies. For example, if you had a RLS function (say)
create or replace function rls_func ... as
begin
return 'hiredate is not null';
end;
/
then the trace file created when you ran "select * from emp" would show
Policy function: RLS_FUNC
RLS view :
SELECT "EMPNO","ENAME","JOB","MGR","HIREDATE","SAL","COMM","DEPTNO" FROM "CHA"."EMP" "EMP" WHERE (hiredate is not null)
Then you can focus your tuning efforts on that *true* SQL