That might still be irrelevant. If only one core can talk to the network card, then there is no parallelisation. It is a classic problem with loads of hardware that is either cheap, or has broken firmware.
You can check by having a look at /proc/interrupts.
Generally IPFire can easily transfer 10 GBit/s if hardware permits. Even with an Intel Atom processor this is possible.
Ok I installed nginx on ipfire and downloaded a 10GB file with wget and get 1.04GB/s so hardware on LAN side should be good right? - WAN is running over SFP+ and LAN over BASE-T
No. Keep in mind that there are limits depending on Roundtrip time/ping. A network with 80ms ping has a theoretical limit of 1.5Gbit for one TCP Stream. (If the buffers and segment sizes are at default settings)
Arne is right. You might not get a 10 GBit/s link saturated using a single TCP connection. However, the NIC is not helping here either because it further increases latency (because you will have to wait that the first processor core becomes available to handle packets).
You could try a firmware update, another NIC, or replace the whole machine.
Just for reference, our IPFire Enterprise Appliance performs load-balancing over all processor cores and therefore is very power efficient:
The other appliances do this too, which is why they all have relatively small processors because a lot of work is already done by the network controller.