SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    13274, 13275, 13276, 13279, 13277, 13278
  ) 
  AND feature_id IN (
    595, 628, 627, 624, 625, 553, 626, 622, 
    623, 636, 637
  ) 
  AND lang_code = 'en'

Query time 0.00360

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "5.94"
    },
    "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": "PRIMARY",
      "used_key_parts": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "key_length": "15",
      "rows_examined_per_scan": 13,
      "rows_produced_per_join": 13,
      "filtered": "100.00",
      "using_index_for_skip_scan": true,
      "cost_info": {
        "read_cost": "4.64",
        "eval_cost": "1.30",
        "prefix_cost": "5.94",
        "data_read_per_join": "10K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`test2_gaseus`.`cscart_product_features_values`.`product_id` in (13274,13275,13276,13279,13277,13278)) and (`test2_gaseus`.`cscart_product_features_values`.`feature_id` in (595,628,627,624,625,553,626,622,623,636,637)) and (`test2_gaseus`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id feature_id variant_id
13274 553 1912
13275 553 1912
13276 553 1912
13277 553 1912
13278 553 1912
13279 553 1912
13274 595 1904
13275 595 2451
13276 595 1787
13277 595 1905
13278 595 1900
13279 595 1462
13274 622 2559
13275 622 2559
13276 622 2559
13277 622 2559
13278 622 2559
13279 622 2559
13274 623 2524
13275 623 2524
13276 623 2524
13277 623 2524
13278 623 2524
13279 623 2524
13274 624 1694
13275 624 1694
13276 624 1694
13277 624 1694
13278 624 1694
13279 624 1694
13274 625 2122
13275 625 2122
13276 625 2122
13277 625 2122
13278 625 2122
13279 625 2122
13274 626 1917
13275 626 1917
13276 626 1917
13277 626 1917
13278 626 1917
13279 626 1917
13274 627 2290
13275 627 2290
13276 627 2290
13277 627 2290
13278 627 2290
13279 627 2290
13274 628 1660
13275 628 1660
13276 628 1660
13277 628 1660
13278 628 1660
13279 628 1660
13274 636 2344
13275 636 2344
13276 636 2344
13277 636 2344
13278 636 2344
13279 636 2344
13274 637 2345
13275 637 2345
13276 637 2345
13277 637 2345
13278 637 2345
13279 637 2345