Data Loader SOAP API versus Bulk API settings for large loads
Key Takeaway
With Bulk API in Data Loader, a blank CSV cell doesn't clear a field on update, so put #N/A in the cell instead. Loads that are likely to cause lock contention, such as updating ownership for records with private sharing, need serial mode.
Data Loader can send records to Salesforce through the SOAP-based API, which it uses by default, or through Bulk API or Bulk API 2.0. The Bulk options are built to load or delete large numbers of records faster. Choosing one also changes how batch size, blank values, oversized values, record locking and deletes work, so the choice affects more than speed.
SOAP-based API and Bulk API
By default, Data Loader processes records with the SOAP-based API. You can switch it to Bulk API or Bulk API 2.0 in Data Loader's settings.
Bulk API and Bulk API 2.0 are optimized to load or delete many records asynchronously. They are faster than the SOAP-based API because they process records in parallel and make fewer network round-trips. Turn one on when you need to insert, update, upsert or delete a large number of records faster than the default SOAP-based API allows.
Bulk API 2.0 supports all OAuth 2.0 flows that other Salesforce REST APIs support. Its design is more consistent and better integrated with other Salesforce APIs. Bulk API 2.0 also has the advantage of future innovation.