I recently built a new machine to try out ipfire as alternative to pfsense. The machine is an i5 -12600h , 16gb ram and was used with no issues on pfsense. My ISP provides and 8GB/8GB connection. I had no issues getting 8gb/7gb on 10gb clietnts. Having setup ipfire with no qos or filtering etc I can only get around 5gb down and 4gb up on my 10gb clients.. The nic for the wan is an X540 which was what I used before.
This could have a thousand reasons. We have plenty of IPFire setups out there that easily reach 10 GBit/s. We even have a few that have 100 GBit/s and I have one that even has 200 GBit/s. Basically it is not the software.
It will probably be some hardware limitation. Bus bandwidth, CPU (although that looks like a beefy one).