Do you qualify for ABTC? That does help avoid the long immigration lines.
Busan can get backed up as well, although nothing on the scale of ICN. I don't see any relief from this unless there is another health scare such as MERS in summer 2015. Also, barring any delays the new Terminal is expected to be operational by the end of 2016.
In the meantime, if you have to get in line then go to the Korea Citizen line. If they question you, just tell them that the Customs official told you to get in that line.