Microsoft throttles their Azure Apps to ensure a consistent experience for all users. To improve performance and to minimize throttling, register multiple Azure apps.

For Exchange Online the recommendation is 5 Azure Apps for 5,000 mailboxes and every time an additional 1k mailboxes are added, register 1 additional app.

https://docs.metallic.io/metallic/93448_registering_exchange_online_with_azure_01.html

Commit your backup and restart and you should see a faster overall backup window.