RS600 Unusable VPN connection

RS600 FW Version 1.0.1.74
I am unable to transfer files from an SMB share over a VPN connection.
The speed is very slow and the file transfer fails for anything over about 30 MBytes.
Remote desktop connections are fine, it seems to fail when large amounts of data need to transfer over the VPN.

I'm connecting to my office IPSEC VPN using the Windows 11 client, the endpoint is a Draytek 2862 router.

My home connection is 1Gbit download, 100Mbit upload (Virgin Media), the office has 200Mbit upload and download speeds.

I have tried disabling DOS protection, Armour and even temporarily put a machine in the DMZ but it makes no difference.

If I reconnect my old TP-Link Archer C3200, the VPN works flawlessly.

I have a case logged with support. Anyone else had this?