iPerf on Fritzbox stock firmware

iPerf is a great cli tool to measure raw network throughput between two devices. I often use it with OpenWrt and while talking about AVMs Fritzbox routers during a Freifunk Berlin meeting, Nicolas mentioned that there is a little-known support interface with their stock firmware at https://fritz.box/support.lua that offers starting an iPerf server – look for Durchsatzmessungen. Very handy.
Note that on client side, parameter -p 4711 is required for the non-standard port.