SELECT 
  cscart_products_categories.product_id, 
  GROUP_CONCAT(
    IF(
      cscart_products_categories.link_type = "M", 
      CONCAT(
        cscart_products_categories.category_id, 
        "M"
      ), 
      cscart_products_categories.category_id
    )
  ) AS category_ids, 
  product_position_source.position AS position 
FROM 
  cscart_products_categories 
  INNER JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
  AND cscart_categories.storefront_id IN (0, 1) 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
  LEFT JOIN cscart_products_categories AS product_position_source ON cscart_products_categories.product_id = product_position_source.product_id 
  AND product_position_source.category_id = 6 
WHERE 
  cscart_products_categories.product_id IN (
    96111, 89233, 96931, 96923, 96621, 92921, 
    96897, 96889, 96826, 95940, 95939, 
    95938, 95937, 95936, 95935, 95934, 
    95933, 95932, 95931, 95930
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00053

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_products_categories range PRIMARY,pt pt 3 21 Using where; Using index
1 SIMPLE cscart_categories eq_ref PRIMARY,c_status,p_category_id PRIMARY 3 toolboxdigitalshop_dev.cscart_products_categories.category_id 1 Using where
1 SIMPLE product_position_source eq_ref PRIMARY,pt PRIMARY 6 const,toolboxdigitalshop_dev.cscart_products_categories.product_id 1

Result

product_id category_ids position
89233 6M 0
92921 6M 0
95930 6M 0
95931 6M 0
95932 6M 0
95933 6M 0
95934 6M 0
95935 6M 0
95936 6M 0
95937 6M 0
95938 6M 0
95939 6M 0
95940 18,6M 0
96111 6M 0
96621 6M 0
96826 6M 0
96889 6M 0
96897 6M 0
96923 6M 0
96931 6M 0