Two hours extra seems unusually longer. When I use to fly the NAT Tracks to JFK, to be stuck behind a 340 was agony. ( 747 .86). I have not flown a 340 but I think they cruise around .82 The real problem is they climb like a Cessna 402 full of Pacific islanders. If your original booking was a 380 or a 747-8 I can see an hour difference but not two.