The Odoo UI makes XMLRPC calls and has batching for large datasets so unless you want finer grained control over how many records you import at once they will both give you the same outcome.

In both cases, you are bound by the time a worker is allowed to tie up Server resources: 

limit-time-real (defaults to 2 minutes)

https://www.odoo.com/documentation/18.0/developer/reference/cli.html#cmdoption-odoo-bin-limit-time-cpu

SQL updates may also be an option https://www.odoo.com/documentation/18.0/applications/essentials/export_import_data.html#export-import-different-tables-from-an-sql-application-to-odoo

Be aware that SQL bypassses all business logic and security so be sure you understand this approach and test. It is a very advanced method and can lead to data inconsistency if not done properly.