Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 11:10 and arrives at Jeju City International Airport at 12:25.
The flighttime is about 1h 15m. The distance travelled ("as the bird flies"): 271 Km. So the average speed is roughly 217 Km.
The best day to arrive at Seoul is Saturday and it would be best to avoid Sunday, as prices are on average 142.76% higher than on Saturday. (Average prices, based on 605939 datapoints.)
We have a lot more statistics for flights KE 1083 and between Seoul and Jeju City.