PPPoE is cpu hungry, but honestly most modern routers should be fine dealing with a gig PPPoE connection - I had no issues getting full speeds with pfsense and an N100 for e.g.

Why not wait and see how it performs when you have the connection.. if it's not quick enough, consider alternatives then. I don't know about the unifi implementation, but PPPoE is typically a single-threaded applciation, so throwing more cores at it won't help. you might be better off asking teh que...