Sandbox

SELECT 
  c.parent_id, 
  sm.search_phrases, 
  c.category_id 
FROM 
  cscart_categories AS c 
  LEFT JOIN cscart_ab__search_motivation AS sm ON sm.category_id = c.category_id 
  AND sm.lang_code = 'ru' 
WHERE 
  c.id_path LIKE "%573%" 
  AND (
    sm.search_phrases != "NULL" 
    OR c.level = (
      SELECT 
        level 
      FROM 
        cscart_categories 
      WHERE 
        category_id = 573
    )
  ) 
  AND c.company_id = 1 
  AND c.status = "A"

Query time 0.00092

Explain
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	PRIMARY	c	ALL					882	Using where
1	PRIMARY	sm	ref	PRIMARY	PRIMARY	3	stroysan.c.category_id	1	Using where
2	SUBQUERY	cscart_categories	const	PRIMARY,p_category_id	PRIMARY	3	const	1

Result

parent_id	search_phrases	category_id
0		573