SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    277935, 277936, 277937, 277938, 277939, 
    277940, 277941, 277942, 277943, 277944, 
    277945, 277946, 277947, 277948, 277949, 
    277950, 277951, 277952, 277953, 277954
  ) 
  AND feature_id IN (1004) 
  AND lang_code = 'en'

Query time 0.00158

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.06"
    },
    "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": "lang_code",
      "used_key_parts": [
        "lang_code",
        "feature_id",
        "product_id"
      ],
      "key_length": "12",
      "rows_examined_per_scan": 20,
      "rows_produced_per_join": 20,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "5.06",
        "eval_cost": "4.00",
        "prefix_cost": "9.06",
        "data_read_per_join": "15K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "lang_code"
      ],
      "attached_condition": "((`webmarco`.`cscart_product_features_values`.`feature_id` = 1004) and (`webmarco`.`cscart_product_features_values`.`product_id` in (277935,277936,277937,277938,277939,277940,277941,277942,277943,277944,277945,277946,277947,277948,277949,277950,277951,277952,277953,277954)) and (`webmarco`.`cscart_product_features_values`.`lang_code` = 'en'))"
    }
  }
}