I have the same problem. My line speed is 10 mbps.
When I use my old fritzbox router, the download rate matches the line speed,
with ZS the download rate is 5-6 mbps.
My ZS is running on an AMD E4200+ (2×2.2GHz). The cpu usage is about 2%. Both network interfaces are 100mps.
I think the hardware I use is able to handle much higher speeds. I already checked the settings and can’t find an explanation.
What can I do to find the cause?