Slow Data Throughput using Robocopy and FSx Filesystem for Windows File Servers

I could use some guidance on improving data transfer speeds while using Robocopy to copy data into an FSx filesystem. Some details

Data source is outside AWS cloud (However, transfer tests from an EC2 instance in same subnet was below par)

No Direct Connect but provisioned network throughput into AWS is not a bottleneck (confirmed with Diskspd)

FSx Filesystem uses SSD drives (yeah I tried HDD but SSD is not pulling its weight either.)

Robocopy parameters: disabled ACLs capture to rule out permissions errors; tried thread counts of 50 through 128; output redirected to log file; no restartable mode

Data throughput looks promising at the start but tails off after about an hour.

FS filesystem dashboard occasional generates a warning about Disk IOPS utilization (SSD); recommends I increase storage capacity even though there is plenty of free space after transfer is complete!

I tried DataSync. Performance was poor (I really did not expect much from an appliance acting as a data relay.)

Can anyone vouch for Robocopy as a tool for bulk data migrations into FSx from outside the AWS cloud?