SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 285000 
  AND lang_code = 'en'

Query time 0.00059

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "594.00"
    },
    "table": {
      "table_name": "cscart_product_features_values",
      "access_type": "ref",
      "possible_keys": [
        "lang_code",
        "product_id",
        "idx_product_feature_variant_id"
      ],
      "key": "product_id",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 495,
      "rows_produced_per_join": 24,
      "filtered": "4.94",
      "index_condition": "(`webmarco`.`cscart_product_features_values`.`lang_code` = 'en')",
      "cost_info": {
        "read_cost": "495.00",
        "eval_cost": "4.89",
        "prefix_cost": "594.00",
        "data_read_per_join": "18K"
      },
      "used_columns": [
        "feature_id",
        "product_id",
        "variant_id",
        "value",
        "value_int",
        "lang_code"
      ]
    }
  }
}

Result

feature_id variant_id value value_int
600 7726
608 64744
609 1486
619 1542
621 7130
622 7133
860 64682
1012 64571
1013 64560
1015 64743
1016 64599
1017 64608
1019 64666
1129 67595
1134 67780