See https://forum.openwrt.org/t/comparative-throughput-testing-including-nat-sqm-wireguard-and-openvpn/44724 the second x86 (AMD GX-412TC) is an APU IIRC, so in theory an APU might be able to traffic shape at your bandwidth...
I concur with @dlakelan, this might indicate one CPU running at maximum. (Potentially caused by packet steering).
it is either a dual or more likely a quad code SoC.
I believe htop is available and will report/display load per core (opkg update; opkg install htop
).