What is the most efficient API for loading approximately 3 million records into Salesforce?

The Bulk API is specifically designed to handle large volumes of data efficiently. It operates asynchronously, allowing users to upload and manage large datasets without the limitations that might be present in other APIs, such as payload size or the number of calls per day. For a dataset of approximately 3 million records, the Bulk API is ideal because it can process large batches of records in a single API call, significantly reducing the time required to complete the data loading process.

The Bulk API also optimizes the data loading process by compressing the data and handling retry logic internally. This is particularly beneficial for large imports, as it minimizes the impact of potential failures during the loading process and ensures that data can be ingested quickly and reliably.

In contrast, the other APIs, such as SOAP and REST, are generally better suited for smaller sets of records and may not be as efficient when dealing with millions of records due to their synchronous nature and limits on the size of individual requests. The Streaming API is primarily used to subscribe to and receive real-time updates from Salesforce rather than for bulk data loads, making it unsuitable for the specific requirement of loading large amounts of data.