Site-to-site performance issues

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Site-to-site performance issues

DS1821+, DS1821+, DS1621+
  1. RT2600ac
Operating system
  1. Windows
Mobile operating system
  1. Android

I've recently installed two RT2600ac - one at my home and another at a remote location.
I have a Synology NAS at each location (DS1821+) to provide offite backup
VPN plus has been set up with site-to-site VPN between the two. The connection works and is stable.
My home internet is 650/550 mbps, whilst the remote is 500/75
The issue I have is that I'm getting much slower transfer times than I would expect. I've tested the following three scenarios with results.

1. Synology Drive between the two NAS: I can consistently get approximately 35 MB/s or 280 mbps when syncing a large multi GB file from local to remote NAS; whilst approximately 6 MB/s or 48 mbps for same file remote to local NAS.
2. Synology Snapshot Replication: seems to max out around 7.5 MB/s or 60 mbps local to remote
3. File transfers NAS to NAS using DSM, or from Windows 10 machine to remote NAS via windows explorer also maxes out around 7.5 MB/s or 60 mbps
The Synology drive sync performance is great and shows that the internet connection is a good one and that Site-to-site VPN performs extermely well intrinsically, with relatively little overhead even with the encryption.
What I don't get is why there is such a performance drop with Synology Snapshot replication or simple file transfers. I'm troubleshooting with tech support, but wanted to get any views from this group.

Of additional note, prior to using a Synology router at the remote location I had a standard ISP router and installed Zerotier on both NAS to enable transfers and this performed similarly for Snapshot replication i.e. around 7.5 MB/s

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