Login with your credentials: https://vault.readynas.com
Goto to: Backup Settings -> Manage Backups -> "Mailbox icon". See below.
You can observe the upload usage per 24h by clicking. Reports -> Account Usage.
Here is the results that I got:
Default: 8.25-1.66 = 6.59 GB/24h => 0.078 MB/s
Set to 100%: 27.9-17.2 = 10.7 GB/24h => 0.13 MB/s
The improvement is 1.6 times faster. Not a huge improvement but better. My Internet connection is 10 Mbit/s (theoretically 1.25 MB/s) for outgoing data so it should not limit the speed in this case.