SELECT 
  pfv.variant_id, 
  pfv.position, 
  pfvd.variant 
FROM 
  cscart_product_feature_variants AS pfv 
  INNER JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfv.variant_id = pfvd.variant_id 
  AND pfvd.lang_code = 'en' 
WHERE 
  pfv.variant_id IN (
    6967, 6968, 6969, 6970, 6971, 6972, 6973, 
    6974, 7165, 6427, 6426, 6425, 6428, 
    6424, 7164
  )

Query time 0.00048

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "pfvd",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "rows": 15,
      "filtered": 80,
      "index_condition": "pfvd.variant_id in (6967,6968,6969,6970,6971,6972,6973,6974,7165,6427,6426,6425,6428,6424,7164) and pfvd.lang_code = 'en'"
    },
    "table": {
      "table_name": "pfv",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "ref": ["herbaria_cscart_4_18.pfvd.variant_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}