@Igor disabling tx offload on the USB ethernet (lan0) seems to solve the problem.
ethtool -K lan0 tx off
And I can't see extra CPU load or appreciable bandwidth drop with this. Great!
To make it work at boot, i changed /etc/udev/rules.d/70-rename-lan.rules as follows
giox069 got a reaction from stavoltafunzia in NanoPi R2S: lan0 goes offline with high traffic