I realise this is an old post, but thought a current update still makes sense.
I just started syncing to an offsite SFTPGo server. ~230,000 blobs, 5.5 TB, 1GBit/s down, 50 MBit/s up, on both ends the repository resides on an external spinning drive. Blob listing initially took up to 2 hours. For blob listing, --parallel is irrelevant (it affects file upload), changing --list-parallelism makes all the difference. With --list-parallelism 50, blob listing now takes 14 minutes. Above that, the connection is aborted in my case.
Hope this helps someone!