My setup is Vodafone Internet 1000/50 -> Technicolor TC4400 -> Fritzbox4040 with openwrt 19.7.7 -> Laptop(via LAN)
On the Laptop i can 250/50 maximal. i tried a speedtest package on the fritzbox which could get 250-300 in download. Upload speed is no problem at all.
I recently switched to the fritzbox4040, so i know that it was possible to get the full download speed of 1000Mbit/s. I also tested at weird times, like 2 am, 7am and 22pm just to be sure that its not a bad timing (running Cable).
I found some thread (Fritz!Box 4040 experiences - #2 by tomtom) where someone suggests enableing softaware nat offload, setting the CPU governors and installing irqbalance. I tried all of that but it doesn't seem to have an effect (the thread is a year old, thats why i didn't post there).
I did a factory reset on 19.7.7, but also tried 18.06 and the rc1 from 21.02 - all with standart configuration as well as the tweaks aabove. I mostly used the standart configuration. Rounter with NAT. No fancy VLAN settings.
Do someone get more than the 250Mbit/s download ? If so, what was your trick ? what did you do ?