I am a Linux and netorking noob so please forgive me if I am asking a basic question.

I have a Linode that is under the 'Linode 512' plan that I don't think is available any more. It runs CentOS 6.6. I recently followed the instructions in the Linode library about setting up a VPN. When I got it all going I ran a test through speedtest.net to compare its download speed to when I access the net just through my normal ISP (Virgin Media). Over a period of time I observed that the upload speed was always around 11.6Mbps but the download speed fluctuated wildly between 6.4Mbps and 75.9Mbps. When I access the net through my ISP only the download speed is constantly around 157Mbps and the upload around 12.2Mbps.

I would like to know if there is anything I can do to improve the performance/stability of my Linode as a VPN?

BTW - There is a pending upgrade for the Linode that will upgrade it to a 'Linode 2GB' plan.

If your VPN is using TCP, and supports UDP, use that instead. It provides a substantial performance boost.

try adding values to your server.conf anf client.ovpm

server:

verb 0
sndbuf 393216
rcvbuf 393216
push "sndbuf 393216"
push "rcvbuf 393216"
mssfix 1360
tcp-nodelay
tun-mtu 1500

Client.ovpn


sndbuf 393216
rcvbuf 393216


these tweaks, maximizes my connection on my openvpn.

The first thing you can do to improve performance is to take the upgrade. Not only will it quadruple your RAM and improve other specs (including networking throughput), it will put you on newer hardware (SSD, KVM). The newer hardware also has better networking.