I have the RT-AC68U and this setting was the culprit behind CPU 1 being pegged at 100%. Disabled it and CPU usage immediately dropped to a few percent. No idea why it only became an issue today.
Old thread but it's one of the first to come up when I googled "Asus RT-AC68U cpu 100%". I registered an account here just to post my solution in case it helps someone in future.
I noticed on the WAN page that "Enable VPN + DHCP Connection" (under the "Special Requirement from ISP" section) was...