SQL> create table t (id integer, val char(80));

Table created.

SQL> declare
  2    st pls_integer := dbms_utility.get_time;
  3  begin
  4    for i in 1..10000 loop
  5      insert into t values(i, 'A');
  6    end loop;
  7    update t set val='B';
  8    delete t;
  9    dbms_output.put_line('No audit done in '||(dbms_utility.get_time-st)/100||'s');
 10  end;
 11  /
No audit done in 2.86s

PL/SQL procedure successfully completed.

SQL> commit;

Commit complete.

SQL> drop table t purge;

Table dropped.

SQL> create table t (id integer, val char(80));

Table created.

SQL> audit insert, update, delete on t;

Audit succeeded.

SQL> declare
  2    st pls_integer := dbms_utility.get_time;
  3  begin
  4    for i in 1..10000 loop
  5      insert into t values(i, 'A');
  6    end loop;
  7    update t set val='B';
  8    delete t;
  9    dbms_output.put_line('Audit done in '||(dbms_utility.get_time-st)/100||'s');
 10  end;
 11  /
Audit done in 6.71s

PL/SQL procedure successfully completed.

SQL> commit;

Commit complete.

SQL> noaudit insert, update, delete on t;

Noaudit succeeded.

SQL> drop table t purge;

Table dropped.

SQL> create table t (id integer, val char(80));

Table created.

SQL> create table a as select * from dba_audit_trail where 1=0;

Table created.

SQL> create or replace trigger trg_t 
  2  before insert or update or delete on t
  3  begin
  4    insert into a 
  5          (os_username, username, userhost, terminal, timestamp, owner, obj_name,
  6           action, action_name, sessionid, os_process, entryid, statementid, returncode)
  7    select osuser,      username, machine,  terminal, sysdate,   user,  'T',
  8           0,      'MY_ACTION', sid,       process,    0, 0, 0
  9    from v$session where sid = sys_context('userenv','SID');
 10  end;
 11  /

Trigger created.

SQL> declare
  2    st pls_integer := dbms_utility.get_time;
  3  begin
  4    for i in 1..10000 loop
  5      insert into t values(i, 'A');
  6    end loop;
  7    update t set val='B';
  8    delete t;
  9    dbms_output.put_line('With trigger done in '||(dbms_utility.get_time-st)/100||'s');
 10  end;
 11  /
With trigger done in 10.33s

PL/SQL procedure successfully completed.

SQL> commit;

Commit complete.