OpenVPN client causing latency and packet loss on non VPN traffic

  • I have an Open VPN client routing traffic from a couple of devices on my LAN, the remainder of the outbound LAN traffic goes out via the normal WAN gateway.

    I have firewall rules on the LAN port to direct traffic coming from the required internal IPs to the VPN gateway, the rules are above my default LAN to Any rule. I then have individual Outbound NAT rules for each internal IP to be routed via the VPN. Traffic appears to be sent and received over the VPN perfectly normally.

    My issue is that when traffic is being sent over the VPN connection, my entire WAN connection experiences spikes in latency and packet loss, affecting all clients on my LAN, to the point that even pinging my cable modem's address is laggy and lossy. I only get the issue when the VPN has traffic going over it, if I shut down the clients using the VPN but leave the VPN connection up everything is absolutely fine.

    I'm running a box with an Atom E3845, it supports AES-NI but using the hardware crypto for the VPN seems to make no difference. My CPU is generally at about 5%-15% during normal use and between 10% - 25% when the VPN is in use.

    I'm not sure where to start figuring out the cause!

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