Flight Incheon International Airport departs from Seoul Incheon International Airport at 22:55 and arrives at Bandar Seri Begawan Brunei International Airport at 03:15.
The flighttime is about 5h 20m. The distance travelled ("as the bird flies"): 2207 Km. So the average speed is roughly 414 Km.
There is an 1 hour time difference between Seoul and Bandar Seri Begawan