The ZyXEL Armor Z2/ NBG6817 is basically the same hardware, so the same applies here:
Only using the OEM firmware (which can offload large parts of the routing in hardware (well, proprietary firmware) to the NSS/ NPU cores).
If you're dealing with this kind of WAN throughput, you're looking for mvebu or x86_64 instead, the closer you get towards 1 GBit/s (and beyond), the stronger the pendulum goes towards x86_64.