Ah, didn’t realize pfSense is the OS, not something that runs on linux. My command examples won’t work for you.

So my first question is how can it be that my little mini J1900 Celeron (2 GHz) with 4 GB RAM cannot handle this bandwith?

• check ethtool for link speed: sudo ethtool enp2s0 | egrep 'Speed|Duplex' Your device name may be different from enp2s0. use ip link to see all devices. if it’s not

Speed: 1000Mb/s
Duplex: Full

then that’s probably a bad sign.

• that is a 10 year old celeron processor. celeron were the budget (a.k.a. cheapest, slowest) class processor at the time. it’s quite likely that it cannot keep up.
• If you still think it’s not CPU directly, use iotop to see if you have I/O bottleneck.