SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    7278, 7351, 7353, 7355, 7357, 7359, 6767, 
    3188, 6760, 6744, 3196, 6749, 3178, 
    3290, 3276, 6824, 3292, 3274, 3285, 
    3286, 6833, 6840
  ) 
  AND feature_id IN (
    625, 624, 628, 595, 641, 637, 636, 627, 
    623, 622, 626, 553
  ) 
  AND lang_code = 'en'

Query time 0.00347

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "59.70"
    },
    "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": 264,
      "rows_produced_per_join": 264,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "33.30",
        "eval_cost": "26.40",
        "prefix_cost": "59.70",
        "data_read_per_join": "204K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`gaseus`.`cscart_product_features_values`.`product_id` in (7278,7351,7353,7355,7357,7359,6767,3188,6760,6744,3196,6749,3178,3290,3276,6824,3292,3274,3285,3286,6833,6840)) and (`gaseus`.`cscart_product_features_values`.`feature_id` in (625,624,628,595,641,637,636,627,623,622,626,553)) and (`gaseus`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}