VPN Throughput - DS218+

Hi I have a DS218+ and I use OpenVPN + TorGuard. My home broadband is a 350Mbps connection.

Without the VPN the NAS saturates the link and I can download at a steady 32MBps but sometimes it hits 37-40MBps

When connected to the VPN my connections top out at around 10-12MBps.

Looking at the CPU when the VPN is on it is around 30-40% so I’m not sure what the bottleneck is?

Is this just a limitation of the NAS?

Is there anything I can do to improve bandwidth when connected to a VPN?