Bandwith loss behind FritzBox 7412

[sentor]

Use OPNsense on Alix APU 2C4 behind FritzBox 7412. The FritzBox handle the internet connection VDSL 100. The LAN Interface (FritzBox) 100Mbit/s is directly connected with the WAN interface on the APU.

I measure the bandwith with iperf3 in two ways:
- 1. directly on the LAN interface of the FritzBox ... got ~ 90 Mbit/s ... measured with a Linux laptop
- 2. on the APU box from commandline ... got ~ 70Mbit/s ...

Any ideas whats the reasons for the delta of 20Mbit/s and how to fix this?

Thanks for hints!