SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    7502, 7503, 7584, 7585, 7622, 7623, 7660, 
    7661, 7698, 7699, 3722, 1439, 5147, 
    5148, 5146, 5145, 1438, 1436, 1437, 
    1435, 1434, 3721
  ) 
  AND feature_id IN (
    627, 623, 622, 626, 553, 625, 624, 628, 
    595
  ) 
  AND lang_code = 'en'

Query time 0.01041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "45.15"
    },
    "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": 199,
      "rows_produced_per_join": 199,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "25.25",
        "eval_cost": "19.90",
        "prefix_cost": "45.15",
        "data_read_per_join": "153K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`gaseus`.`cscart_product_features_values`.`product_id` in (7502,7503,7584,7585,7622,7623,7660,7661,7698,7699,3722,1439,5147,5148,5146,5145,1438,1436,1437,1435,1434,3721)) 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'))"
    }
  }
}