SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    266, 265, 112, 267, 268, 269, 270, 271, 
    272
  ) 
  AND feature_id IN (9, 7) 
  AND lang_code = 'ja'

Query time 0.00061

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_features_values",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY",
        "fl",
        "lang_code",
        "product_id",
        "fpl",
        "idx_product_feature_variant_id"
      ],
      "key": "idx_product_feature_variant_id",
      "key_length": "12",
      "used_key_parts": ["product_id", "feature_id", "lang_code"],
      "rows": 18,
      "filtered": 100,
      "attached_condition": "cscart_product_features_values.product_id in (266,265,112,267,268,269,270,271,272) and cscart_product_features_values.feature_id in (9,7) and cscart_product_features_values.lang_code = 'ja'",
      "using_index": true
    }
  }
}