SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    300, 303, 304, 305, 306, 307, 308, 309, 
    310, 311, 312, 313, 314, 315, 316
  ) 
  AND feature_id IN (550) 
  AND lang_code = 'en'

Query time 0.00078

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.68"
    },
    "table": {
      "table_name": "cscart_product_features_values",
      "access_type": "ref",
      "possible_keys": [
        "PRIMARY",
        "fl",
        "lang_code",
        "product_id",
        "fpl",
        "idx_product_feature_variant_id"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "feature_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 7,
      "rows_produced_per_join": 0,
      "filtered": "2.41",
      "using_index": true,
      "cost_info": {
        "read_cost": "1.28",
        "eval_cost": "0.03",
        "prefix_cost": "2.68",
        "data_read_per_join": "133"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`atulecarter_atul_demo1`.`cscart_product_features_values`.`product_id` in (300,303,304,305,306,307,308,309,310,311,312,313,314,315,316)) and (`atulecarter_atul_demo1`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}