monitor.backfill() in datadrift tutorial is quite time consuming

Hello Microsoft Team,

I am studying datadrift monitoring and following [Azure datadrift tutorial](https://github.com/Azure/MachineLearningNotebooks/blob/master/how-to-use-azureml/work-with-data/datadrift-tutorial/datadrift-tutorial.ipynb) but when I go to `monitor.backfill()` step, it is quite time consuming to execute.

```
from azureml.widgets import RunDetails

backfill = monitor.backfill(dt.datetime.now() - dt.timedelta(weeks=6), dt.datetime.now())

RunDetails(backfill).show()
backfill.wait_for_completion()
```

As you can see in the capture picture. It have been wasting me up to 9h but have not yet completed. My baseline and target data keep the same as example and I run on compute instance with configuration:

```
Virtual machine size
Standard_DS11_v2 (4 cores, 14 GB RAM, 28 GB disk)
```
Can you explain what error happen with this example tutorial?

從 datadrift-tutorial.ipynb 和報告中顯示的 monitor.backfill() 儲存格開始。在指定的 Standard_DS11_v2 計算執行個體上執行相同的六週 backfill，並檢查 RunDetails 輸出和完成行為。完成標準是找出教學課程 backfill 無法在合理時間內完成的原因，或記錄所需的修正。

由索引模型根據 Issue 內容生成。