SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    12378, 12377, 12376, 12375, 12374, 12373, 
    12372, 12371, 12370, 12369, 12368, 
    12367, 12366, 12365, 12364
  ) 
  AND feature_id IN (624, 625, 553, 627, 628) 
  AND lang_code = 'en'

Query time 0.00659

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "6.21"
    },
    "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": 14,
      "rows_produced_per_join": 14,
      "filtered": "100.00",
      "using_index_for_skip_scan": true,
      "cost_info": {
        "read_cost": "4.81",
        "eval_cost": "1.40",
        "prefix_cost": "6.21",
        "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 (12378,12377,12376,12375,12374,12373,12372,12371,12370,12369,12368,12367,12366,12365,12364)) and (`test2_gaseus`.`cscart_product_features_values`.`feature_id` in (624,625,553,627,628)) and (`test2_gaseus`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}

Result

product_id feature_id variant_id
12364 553 1935
12365 553 1935
12366 553 1935
12367 553 1935
12368 553 1935
12369 553 1935
12370 553 1935
12371 553 1935
12372 553 1935
12373 553 1935
12374 553 1935
12375 553 1935
12376 553 1935
12377 553 1935
12378 553 1935
12364 624 2381
12365 624 2381
12366 624 2381
12367 624 2381
12368 624 2381
12369 624 2381
12370 624 2381
12371 624 2381
12372 624 2381
12373 624 2381
12374 624 2381
12375 624 2381
12376 624 2381
12377 624 2381
12378 624 2381
12364 625 2408
12365 625 2409
12366 625 2410
12367 625 2411
12368 625 2388
12369 625 2408
12370 625 2409
12371 625 2410
12372 625 2411
12373 625 2388
12374 625 2408
12375 625 2409
12376 625 2410
12377 625 2411
12378 625 2388
12364 627 2378
12365 627 2378
12366 627 2378
12367 627 2378
12368 627 2378
12369 627 1836
12370 627 1836
12371 627 1836
12372 627 1836
12373 627 1836
12374 627 1954
12375 627 1954
12376 627 1954
12377 627 1954
12378 627 1954
12364 628 1712
12365 628 1712
12366 628 1712
12367 628 1712
12368 628 1712
12369 628 1712
12370 628 1712
12371 628 1712
12372 628 1712
12373 628 1712
12374 628 1712
12375 628 1712
12376 628 1712
12377 628 1712
12378 628 1712