Just did this a few days ago, it was quick and easy. Passengers connecting to other flights go through a security screening, basically they check your boarding pass and run you through x-ray and metal detector. Very lenient too, no need to remove your shoes or take out your liquids. For me the walk was very short from my arrival gate to the security location, but I can't say how many of these security locations for connecting passengers there may be in the airport, as it was my first time. Also, I arrived very early in the morning, so there were not many other passengers to contend with. After the security screening, you head directly up an escalator to the gate area.

Overall, I found ICN to be very efficient and an easy layout to navigate. It's a beautiful facility.