@dfunkt how about this tweak?
Connect to your router via SSH and run these commands:
opkg update
opkg install irqbalance
uci set network.globals.packet_steering=1; uci commit network
uci set irqbalance.irqbalance.enabled=1; uci commit
# What you can try is set this in your /etc/rc.local:
echo performance > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
echo e > /sys/class/net/eth0/queues/rx-0/rps_cpus
echo 2 > /sys/class/net/eth0/queues/tx-0/xps_cpus
# or variant 2
echo performance > /sys/devices/system/cpu/cpufreq/policy0/scaling_governor
echo f > /sys/class/net/eth0/queues/rx-0/rps_cpus
echo 2 > /sys/class/net/eth0/queues/tx-0/xps_cpus