Oracle optimiza OR + IN a OR + EXISTS, que es muy lento

Podría transformar su subconsulta en una unión:

SELECT * 
FROM application_log log
    LEFT JOIN transaction transaction ON log.tag_value=transaction.id AND transaction.sale_id = 'xxx'
WHERE log.tag_value = 'xxx' OR transaction.id IS NOT NULL
ORDER BY log.log_date ASC;