Flight Incheon International Airport departs from Seoul Incheon International Airport at 17:55 and arrives at Phuket Phuket International Airport at 22:15.
The flighttime is about 6h 20m. The distance travelled ("as the bird flies"): 3274 Km. So the average speed is roughly 517 Km.
There is an 2 hour time difference between Seoul and Phuket