We also had some issues with restore speed using File Level Restore.
As a workaround, I did the following:
1. Start Instant VM Recovery of the VM, but do not power on.
2. Edit setting of the VM where you want to restore the files,
add the disk of the restored VM (in Veeam NFS Datastore)
3. Now the backupdata is available in the VM, and you can restore with robocopy.
The big advantage is that restore is done locally, so there is no network traffic.
Also, if Veeam server is in another VLAN than the VM where you want to restore to, traffic doesn't need to be routed...
I wonder if it isn't possible for Veeam to implement this in the Veeam GUI as an extra option when using File level restore...