What do you mean by modern hardware J0hn?
I suppose if you discount 3rd party opensource solutions it might be negligible, but I have seen reports where a CPU can handle 6 gbit/sec without PPPoE, and collapses to under 600mbit/sec (tenth of speed) with PPPoE, and the CPU isnt some obsolete 10 year old atom CPU.
I think one of the issues is as Alex said with getting it to multithread in opensource drivers, not sure if there is anything else on top of that as the cause?
Does talk talk residential over cityfibre use PPPoE?