I am currently running a Vigor2865 on 4.4.6.1_BT connected to talktalk adsl broadband.
I have followed this  guide  and it working correctly however my speeds are slower than when I have the vpn disconnected 40-50mbps rather than 70-80mbps.
I have also found that when running a ping 1.1.1.1 -t command on a pc if another device saturates the network pings begin to fail. the router itself also shows around 90% utilisation.
Any suggestions?

Do you have Hardware Acceleration enabled?

Yes.
Currently enabled for NAT TCP/UDP and IPsec TCP/UDP

Also, what do you mean by 90% utilisation? Router CPU?

Hi John ,

I have followed this  guide  and it's working correctly, however, my speeds are slow...

Try changing the encryption level from AES256 to AES192... ...if NordVPN allows it?

Unfortunately, the Vigor routers (well, most of the ones I have encountered) bog down when processing encrypted tunnels at AES256 with busy, continuos streams. They seem to be ok with AES192 though, so try that and see if it helps.