I recently read somewhere that one of the differences between ipfire and pfsense/opnsense is that ipfire uses all available cores and achieves greater throughput whereas the others use a single core and achieve slower throughput.
Is this true? If I understand this correctly ipfire must perform better overall but some others may include more options and features.
Yes, this is true. Some network adapters do not support this, but if they do, then all process cores will handle packets.
Regardless of that, services like the Intrusion Prevention System will always use all cores to scan packets.
We should get the best out of your hardware, because we optimise IPFire to run on really small single-board computers, but gain a lot of those optimisations on large servers with multiple 10G interfaces, too.