Flight Incheon International Airport departs from Seoul Incheon International Airport at 21:10 and arrives at Changsha Huanghua International Airport at 23:30.
The flighttime is about 3h 20m. The distance travelled ("as the bird flies"): 1554 Km. So the average speed is roughly 466 Km.
There is an 1 hour time difference between Seoul and Changsha