How can we load millions of records in Salesforce within an hour ?

Use Data Loader when:
You need to load 50,000 to 5,000,000 records. Data Loader is supported for loads of up to 5 million records. If you need to load more than 5 million records, we recommend you work with a Salesforce partner or visit the App Exchange for a suitable partner product.
You need to load into an object that is not yet supported by the import wizards.
You want to schedule regular data loads, such as nightly imports.
You want to export your data for backup purposes.

Use the import wizards when:
You are loading less than 50,000 records.
The object you need to import is supported by import wizards. To see what import wizards are available and thus what objects they support, from Setup, enter Data Management in the Quick Find box, then select Data Management.
You want to prevent duplicates by uploading records according to account name and site, contact email address, or lead email address.

For this Scenario you need to use Third Party tool

Thanks