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'))"
    }
  }
}

Result

product_id feature_id variant_id
511 553 1912
511 595 1789
511 622 2325
511 623 2087
511 624 1705
511 625 2364
511 626 1727
511 627 1659
511 628 1660
512 553 1912
512 595 1789
512 622 2325
512 623 2085
512 624 1705
512 625 2364
512 626 1727
512 627 1659
512 628 1660
513 553 1912
513 595 1789
513 622 2325
513 623 2083
513 624 1705
513 625 2364
513 626 1727
513 627 1659
513 628 1660
3672 553 1912
3672 595 1789
3672 622 2325
3672 623 2088
3672 624 1705
3672 625 2364
3672 626 1727
3672 627 1659
3672 628 1660
3673 553 1912
3673 595 1789
3673 622 2325
3673 623 2086
3673 624 1705
3673 625 2364
3673 626 1727
3673 627 1659
3673 628 1660
3674 553 1912
3674 595 1789
3674 622 2325
3674 623 2084
3674 624 1705
3674 625 2364
3674 626 1727
3674 627 1659
3674 628 1660
4270 553 1912
4270 595 1789
4270 622 2325
4270 623 2092
4270 624 1705
4270 625 2364
4270 626 1727
4270 627 1659
4270 628 1660
7471 553 1912
7471 595 1748
7471 622 2325
7471 623 2083
7471 624 1705
7471 625 2364
7471 626 1727
7471 627 1659
7471 628 1660
7472 553 1912
7472 595 1748
7472 622 2325
7472 623 2092
7472 624 1705
7472 625 2364
7472 626 1727
7472 627 1659
7472 628 1660
7473 553 1912
7473 595 1748
7473 622 2325
7473 623 2084
7473 624 1705
7473 625 2364
7473 626 1727
7473 627 1659
7473 628 1660
7474 553 1912
7474 595 1748
7474 622 2325
7474 623 2086
7474 624 1705
7474 625 2364
7474 626 1727
7474 627 1659
7474 628 1660
7475 553 1912
7475 595 1748
7475 622 2325
7475 623 2085
7475 624 1705
7475 625 2364
7475 626 1727
7475 627 1659
7475 628 1660
7476 553 1912
7476 595 1748
7476 622 2325
7476 623 2088
7476 624 1705
7476 625 2364
7476 626 1727
7476 627 1659
7476 628 1660
7477 553 1912
7477 595 1748
7477 622 2325
7477 623 2087
7477 624 1705
7477 625 2364
7477 626 1727
7477 627 1659
7477 628 1660