Speed factors will be:
The speed of the NIC and the Switches
The speed of the bandwidth over the WAN or link.
It sounds as if you are doing a VPN over a LAN so with the 1GBPS connection between the two PC’s, 100mbps should be doable. If you are getting slower network speeds than expected check the configuration of the NIC’s, check for errors logging on the NIC’s and switch, ensure everything is full duplex, network teaming working properly, etc. Also check the config of the VPN software you are using, ensure the configs are the same and optimized on both VPN machines.