D. Load all data after deferring sharing calculations.

This recommendation is based on the following considerations:

When loading large volumes of data into Salesforce, sharing calculation can significantly impact the performance and speed of the data load process. Deferring sharing calculations until after the load is complete can improve performance.

Roll-Up summary fields can also impact performance during data loads, especially in private sharing models where calculations are more complex due to sharing rules. By deferring sharing calculations, the system can first efficiently load all the data and then update the Roll-Up summaries and sharing rules in a more controlled and efficient manner.

Should it not be A.

The question states the data will be imported regularly and data is coming from an external source which is the master.

Using external id's to load data results in faster loads as external id's are indexed automatically, right?