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 
FROM 
  cscart_products_categories 
  INNER JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
  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') 
WHERE 
  cscart_products_categories.product_id IN (
    4, 1, 5, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 
    17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 
    27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 
    37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 
    47, 48, 49, 50, 51
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00066

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "sort_key": "cscart_products_categories.product_id",
        "table": {
          "table_name": "cscart_products_categories",
          "access_type": "ALL",
          "possible_keys": ["PRIMARY", "pt"],
          "rows": 267,
          "filtered": 19.10112381,
          "attached_condition": "cscart_products_categories.product_id in (4,1,5,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51)"
        }
      }
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY", "c_status", "p_category_id"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["category_id"],
      "ref": ["smscrm_edev.cscart_products_categories.category_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "(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')"
    }
  }
}