[SP2013 Online] Massive import in SharePoint List

Hello,

We have to import 5 million of entries in a SharePoint list. We have SharePoint 2013 online, in an app.

We tried with .csv and C#, but it's not really fast, and the script stops itself after 10 000/20 000 item's creations.

Do you have any idea for those kind of import to be fast and safe ?

Thanks !