At least on the Gilgamesh server (and I've heard this is true for all NA/EU servers), regardless of your ISP or internet connection or PC specs, there is a large amount of server lag. This manifests itself in many annoying ways, but the easiest way to see it is to simply cast a spell and jump with about 50 remaining on the timer. More often than not, the spell completes anyway. This translates into 500ms of server latency which is pretty terrible. Even half that amount is bad.
Is SE aware of this issue? Is it something they are working on? Right now the game feels sluggish and unresponsive due to this, even on the very best PC on the very best connection.