That's a Fritz with a Gigabit Ethernet switch according your original post.
Failed or broken switch port, failed Ethernet negotiation, wrong switch port configuration, bad cables, incompatile Ethenret port configuration, or unsupported port trunking cand and will cause such low perfromance level.
Check the Fritz' Ethernt switch port config (if configureable).
Don't use port tunking, start with a single RJ45 conneciton only.
Ensure all four pairs of the cable are correct, try an alternate cable.