I recently done some digging around on VPNs and seems no one has bothered to really support Asus merlin, the best and only provider is Astrill vpn.
Am still new to my Ac86, but it does sport 1.8ghz dual core and AES, and AES I believe is the key to getting good speeds while under vpn providers that support AES, I can usually be within 10-20% of my overall broadband speed while under a AirVPN.
But currently AirVPN does not have an merlin plugin like Astrill and you do get port forwarding under VPN and more policy routing and policy routing websites which imo is excellent.
I would have loved to see port forwarding under vpn and especially Policy routing websites under Merlin firmware, I may give astrill vpn a trial since features and support like that is rare amongst VPN providers.