The current engine design requires everything to be double checked by the server, this was designed to stop things like item duping, speed hacks etc... the downside is this back and forth communication to see if each action is possible caused the player lag. The reason for a new engine is probably because they realised that the current one couldn't do what they had planned (this throws more fuel on the conspiracy theory that it was developed outside SE) so now they are doing a new engine designed for ffxiv.

Back and topic Great translation Reinhart that was a huge interview!