Flight Incheon International Airport departs from Seoul Incheon International Airport at 08:05 and arrives at Hanoi Noi Bai International Airport at 11:10.
The flighttime is about 5h 05m. The distance travelled ("as the bird flies"): 2409 Km. So the average speed is roughly 474 Km.
There is an 2 hour time difference between Seoul and Hanoi