SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    7477, 7476, 7475, 7474, 7473, 7472, 7471, 
    511, 3672, 3674, 4270, 513, 512, 3673
  ) 
  AND feature_id IN (
    627, 623, 622, 626, 553, 625, 624, 628, 
    595
  ) 
  AND lang_code = 'en'

Query time 0.00809

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "29.02"
    },
    "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",
      "used_key_parts": [
        "product_id",
        "feature_id",
        "lang_code"
      ],
      "key_length": "12",
      "rows_examined_per_scan": 127,
      "rows_produced_per_join": 127,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "16.32",
        "eval_cost": "12.70",
        "prefix_cost": "29.02",
        "data_read_per_join": "98K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`gaseus`.`cscart_product_features_values`.`product_id` in (7477,7476,7475,7474,7473,7472,7471,511,3672,3674,4270,513,512,3673)) and (`gaseus`.`cscart_product_features_values`.`feature_id` in (627,623,622,626,553,625,624,628,595)) and (`gaseus`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}