Thanks a lot ! This is what I get on heavy load (750Mbps):
root@LEDE:~# cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
1725000
1000000
and:
CPU: 0% usr 6% sys 0% nic 18% idle 0% io 0% irq 75% sirq
I guess CPU could be the bottleneck...