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 (
    6264, 6267, 6268, 6265, 6266, 6418, 6419, 
    6420, 7164, 6333, 6332, 6475, 6476, 
    6477, 6478, 6479, 6480, 6481, 7168
  )

Query time 0.00066

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": 19,
      "filtered": 78.947,
      "index_condition": "pfvd.variant_id in (6264,6267,6268,6265,6266,6418,6419,6420,7164,6333,6332,6475,6476,6477,6478,6479,6480,6481,7168) 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
    }
  }
}