I spent just 25 minutes for Immigration and Customs when my flight from ICN arrived 30 minutes earlier. It was really very fast (and I was using visitor's lane).
In turn, two weeks ago my flight was late for 15 minutes and I&C took more than an hour (mostly Immigration, where we had single lane for Citizens/Visitors, two flights from Asia and one from Caribbean). As a result I missed not only my booked connection, but also 2 possible reroutings.