Hi, not sure if your problem is now fixed but I have a similar issue with vpn and wired client performance with the Ac88u and merlin 384.19.
In my case, the router is OK for a couple days after reboot and then the cpu starts ticking up and stays there even if vpn tunnels aren't that busy. It'll get to a point after 5 days where I must restart or my wired machine (non-VPN client) starts seeing drops and slowness too.
In my case, the router is OK for a couple days after reboot and then the cpu starts ticking up and stays there even if vpn tunnels aren't that busy. It'll get to a point after 5 days where I must restart or my wired machine (non-VPN client) starts seeing drops and slowness too.