Оптимизация запроса not in (select ...

Добрый день.

Прошу помочь в оптимизации запроса Mysql:

select count(distinct(t.em)) as cr from

  (select email as em from object where email<>''

   union all

   select email as em from job_list where email<>''

   union all

   select from_email as em from send_message where from_email<>\'\'

  ) as t where t.em not in (SELECT email FROM send_ignore_list_email)

Не очень быстро работает, 2.4 сек

Проблема имеено в последней выборке

as t where t.em not in (SELECT email FROM send_ignore_list_email)

Вычитал что not in можно заменить на LEFT OUTER JOIN

Сделал следующее:

select count(distinct(t.em)) as cr from

 (select email as em from object where email<>''

  union all

  select email as em from job_list where email<>''

  union all

  select from_email as em from send_message where from_email<>''

) as t

LEFT OUTER JOIN send_ignore_list_email ON email=t.em where email is null 

Тоже не очень быстро работает, 2.37 сек

Помогите пожалуйста