I'm also experiencing this issue using RT-AX88U as main and RT-AC88U as (wired) AIMesh node.
RT-AX88U is running 386.1_2 and RT-AC86U is running stock 9.0.0.4.386_41994 as I read on wiki that stock firmware is preferred for the node.
Issues started the day after upgrading to 386, I was on 384...
I get about 80 mbit out of 300 without VPN.
I'm also concerned about the long term effects of the CPU running at 100%, which shouldn't be the case if hardware acceleration works, according to the article I linked.
I bought a AX88U to use with OpenVPN due to the fact that it's CPU should support hardware acceleration for the encryption.
I got this information from here: https://www.privateinternetaccess.com/blog/hardware-acceleration-is-here-for-routers-using-openvpn/
However, when using the OpenVPN...