LAN to LAN traffic makes CPU load:
I was testing the max full duplex ethernet LAN speed between 2 win10 computers.
As expected got around 930Mbits Tx and simultaneous 930Mbits Rx.
I used iperf3.11 compiled for windows running in both computers, but can try same test running linux in both computers.

Both computers are wired to AX3600 LAN ports and nothing more is connected.
AX3600 has the last Robi build, configured as dumb AP and the radios are disabled.

What I found interesting was that AX3600 cpu load was sometimes around 25% load when the LAN test was running.
I expected the LAN-LAN traffic to be managed by the ax3600 switch locally and not by the router cpu.
A few other times the same test produced lower cpu usage.

Mem: 156256K used, 261272K free, 228K shrd, 0K buff, 21556K cached
CPU:   0% usr   0% sys   0% nic  75% idle   0% io   0% irq  24% sirq
Load average: 0.44 0.18 0.14 3/98 4323
  PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
   10     2 root     RW       0   0%  23% [ksoftirqd/0]
 4323  4301 root     R     1264   0%   0% top -d 1
 2131  2097 network  S     4464   1%   0% /usr/sbin/hostapd -s -g /var/run/hostapd/global
 2132  2098 network  S     4464   1%   0% /usr/sbin/wpa_supplicant -n -s -g /var/run/wpa_suppl
 1929     1 root     S     2572   1%   0% {dnsmasq} /sbin/ujail -t 5 -n dnsmasq -u -l -r /bin/
 2854     1 root     S     2572   1%   0% {ntpd} /sbin/ujail -t 5 -n ntpd -U ntp -G ntp -C /et

EDIT#1: Also one of the computers had a ssh to the router to see the cpu usage.

EDIT#2: Running the same test, now with my build that has HTOP, shows this:

EDIT#3: Same test with my build with htop, but now running linux mint in both computers and iperf3 v3.7 that comes with mint:
htop2