Huge Data upload

Dear Experts,

I got a requirement from my client, having 40,00,000 of customers need to upload into SAP.

What is the best way to do?

According to my knowledge...

LSMW is slow,

BAPI is slow,

Call Traction is slow

Session method

I do not know about API

Could you please suggest me which is the best method to upload 40,00,000 records.