I'm pretty sure the UI lag has alot to do with how SE developed the game's usage of CPU/GPU. I remember FFXI performance was almost more influenced by the CPU's power than the GPU and alot of the UI lag and mechanics feel similar to XI in that regard. I have a feeling SE just ain't awesome at making the most *efficient* engine out there. The previous point about the majority of information being server-side is probably an even bigger factor.
The beta was absurdly slow... this is almost double the speed I remember getting over the summer--I hope they address it eventually.
XI: Onima -- Lakshmi Server
XIV: Santa Cruz -- Selbina Server