After research, it seems there isn’t really an iron-clad guideline. As in X amount of Passmark score will get you Y speed on a PPPoE link. There’s a lot of discussion out there if you go looking, not so much for IPFire but certainly for the likes of Pfsense.
Although from what I can see, you probably want something relatively recent with a clock speed around 2GHz. Like maybe a recent AMD Zen based or an Intel Core based CPU that’s at least capable of 2GHz. Cores don’t matter as much it’s better have 2 good fast cores rather than 4 or more slower ones.
For reference, this is the profile link for my IPFire box. It easily achieves the 1Gbps throughput even with the IPS running over my ISP’s VLAN tagged PPPoE link.
https://fireinfo.ipfire.org/profile/d9529899e467f0f26364b6f14f6d17f13540fe46
This is the kind of frequency the Athlon’s CPU cores hit in my IPFire box over the last month. It hardly ever goes about 2GHz, and certainly nowhere near the max of 3.2GHz the CPU is capable of.
It likely means that the newer embedded low power Ryzen systems would be great for running IPFire with this kind of connection on the Red.