Expected Speed on Google Cloud

[maxpowers]

Hi
We just deployed out OpenVPN on to Google Cloud (GPC) (https://openvpn.net/index.php/access-se ... cloud.html)
Everything went well, and works as expected. However, the speed is not what I expected.

The max we are able to transfer is under 50mb/s. In real world usage when accessing a file server (also on GPC on the same subnet as the OpenVPN) server we are generally seeing a max of 20mb/s.
Neither the file serve or vpn server are maxing out resources, or even coming close to it. The end vpn client testing machines are all 200mb/s+ connections and pretty new machines with plenty of processing power.

My question is, is this what is to be expected?
I just was hoping with google cloud's technical capacity we would be seeing much better results.
For example, a straight speed test on the file server shows it can transfer in and out at 500mb/s+. BUT, running via OpenVPN we can't get even 10% of that, all while resources (like CPU) are all under 25% usage.

Technical details:
The OpenVPN server is on n1-standard-1 (1 vCPU, 3.75 GB memory)
The file server is n1-standard-2 (2 vCPUs, 7.5 GB memory) running Windows 2016. It isn't doing anything else but file serving.
Both are on the Premium data tier, hosted in us-central1-c

Any tips, comments or suggestions would be greatly appreciated!

Thanks