SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    12969, 12970, 12971, 12972, 12973, 12960, 
    12961, 12962, 12963, 12964, 12965, 
    12966, 12967, 12968
  ) 
  AND feature_id IN (
    646, 647, 595, 628, 627, 624, 625, 553, 
    626, 622, 623, 636, 637
  ) 
  AND lang_code = 'en'

Query time 0.07034

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "11.63"
    },
    "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": 34,
      "rows_produced_per_join": 34,
      "filtered": "100.00",
      "using_index_for_skip_scan": true,
      "cost_info": {
        "read_cost": "8.23",
        "eval_cost": "3.40",
        "prefix_cost": "11.63",
        "data_read_per_join": "26K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`test2_gaseus`.`cscart_product_features_values`.`product_id` in (12969,12970,12971,12972,12973,12960,12961,12962,12963,12964,12965,12966,12967,12968)) and (`test2_gaseus`.`cscart_product_features_values`.`feature_id` in (646,647,595,628,627,624,625,553,626,622,623,636,637)) and (`test2_gaseus`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id feature_id variant_id
12960 553 1973
12961 553 1973
12962 553 1973
12963 553 1973
12964 553 1973
12965 553 1973
12966 553 1973
12967 553 1973
12968 553 1973
12969 553 1973
12970 553 1973
12971 553 1973
12972 553 1973
12973 553 1973
12960 595 1904
12961 595 1904
12962 595 1905
12963 595 1905
12964 595 1789
12965 595 1789
12966 595 2499
12967 595 2499
12968 595 1787
12969 595 1787
12970 595 1900
12971 595 1900
12972 595 1899
12973 595 1899
12960 622 2497
12961 622 2497
12962 622 2497
12963 622 2497
12964 622 2497
12965 622 2497
12966 622 2497
12967 622 2497
12968 622 2497
12969 622 2497
12970 622 2497
12971 622 2497
12972 622 2497
12973 622 2497
12960 623 2501
12961 623 2501
12962 623 2501
12963 623 2501
12964 623 2501
12965 623 2501
12966 623 2501
12967 623 2501
12968 623 2501
12969 623 2501
12970 623 2501
12971 623 2501
12972 623 2501
12973 623 2501
12960 624 1803
12961 624 1803
12962 624 1803
12963 624 1803
12964 624 1803
12965 624 1803
12966 624 1803
12967 624 1803
12968 624 1803
12969 624 1803
12970 624 1803
12971 624 1803
12972 624 1803
12973 624 1803
12960 625 2123
12961 625 2123
12962 625 2123
12963 625 2123
12964 625 2123
12965 625 2123
12966 625 2123
12967 625 2123
12968 625 2123
12969 625 2123
12970 625 2123
12971 625 2123
12972 625 2123
12973 625 2123
12960 626 1721
12961 626 1721
12962 626 1721
12963 626 1721
12964 626 1721
12965 626 1721
12966 626 1721
12967 626 1721
12968 626 1721
12969 626 1721
12970 626 1721
12971 626 1721
12972 626 1721
12973 626 1721
12960 627 1659
12961 627 1659
12962 627 1659
12963 627 1659
12964 627 1659
12965 627 1659
12966 627 1659
12967 627 1659
12968 627 1659
12969 627 1659
12970 627 1659
12971 627 1659
12972 627 1659
12973 627 1659
12960 628 1660
12961 628 1660
12962 628 1660
12963 628 1660
12964 628 1660
12965 628 1660
12966 628 1660
12967 628 1660
12968 628 1660
12969 628 1660
12970 628 1660
12971 628 1660
12972 628 1660
12973 628 1660
12960 636 2348
12961 636 2348
12962 636 2348
12963 636 2348
12964 636 2348
12965 636 2348
12966 636 2348
12967 636 2348
12968 636 2348
12969 636 2348
12970 636 2348
12971 636 2348
12972 636 2348
12973 636 2348
12960 637 2345
12961 637 2345
12962 637 2345
12963 637 2345
12964 637 2345
12965 637 2345
12966 637 2345
12967 637 2345
12968 637 2345
12969 637 2345
12970 637 2345
12971 637 2345
12972 637 2345
12973 637 2345
12960 646 2369
12961 646 2370
12962 646 2369
12963 646 2370
12964 646 2369
12965 646 2370
12966 646 2369
12967 646 2370
12968 646 2369
12969 646 2370
12970 646 2369
12971 646 2370
12972 646 2369
12973 646 2370
12960 647 2373
12961 647 2374
12962 647 2373
12963 647 2374
12964 647 2373
12965 647 2374
12966 647 2373
12967 647 2374
12968 647 2373
12969 647 2374
12970 647 2373
12971 647 2374
12972 647 2373
12973 647 2374