PPPoE as a protocol is rather demanding, but even without PPPoE, 500 MBit/s is quite a lot for most common 'plastic routers'. We're only slowly seeing devices entering this performance region (e.g. mt7622bv or filogic 820/ 830 might be able to do this, ipq807x is probably very borderline).
While it is getting a tad old, this post is still correct and on topic:
Personally I am using x86_64 (old Atom j1900) for 400/200 MBit/s (without PPPoE, but using sqm/cake) - and it's a breeze, as everything just works and has plenty of performance to spare (today I'd probably look at ).