Ok, I managed to extract the files with help of this thread:
I extracted the files from the latest FritzBox 3370 image. I also get a decent speed now.
How do I know if SMP is enabled or if I can use flow offloading? Using option flow_offloading 1
in the firewall config did not work, nor did
uci set firewall.@defaults[0].flow_offloading=1; uci commit firewall