Most of the dev's suffer from ptsd and night-horrors... (What?)
The complexity of the emulator probably rivals that of a modern cpu.
That said, a good place to start exploring the code would probably be here:
https://github.com/EQEmu/Server/blob...ent_packet.cpp
This is where post-protocol handling is performed for incoming client (zone) packets. It should give you a decent idea of what's going on
and what to look for next.
You can always pop in irc (link provided in Akkadius's post) if you have any questions.