I'm sorry if someone has already said this, but I only glanced over the few pages this thread has created, but simply put:
When FFXIV, FFXI, WoW, Aion, Archlord, Moshi Monsters or
whatever send information between your computer and the servers they use 'Packets'. These packets contain the necessary data for the server to do what you expect, this is a number of things:
Text, Movement, Character Data, Position, Rotation, etc.
ALL of your characters unique information that is constantly being changed is being sent in these 'packets', so whenever you start adding in more character customization options, armour pieces, etc. the packets become heavier as more data is added, which would in turn create 'lag' as the packets would require more time to transfer between yourself and the servers.
TLDR
The more you add, the more strain it puts on the servers creating lag.
Source:
http://en.wikipedia.org/wiki/Network_packet
http://searchnetworking.techtarget.c...inition/packet