Flight Incheon International Airport departs from Seoul Incheon International Airport at 14:55 and arrives at Shenzhen Shenzhen Bao'an International Airport at 17:45.
The flighttime is about 3h 50m. The distance travelled ("as the bird flies"): 1621 Km. So the average speed is roughly 423 Km.
There is an 1 hour time difference between Seoul and Shenzhen