The overall server lag that occurs on certain times is a different issue than the lag people talk about. I had times where I can see Demon Wall finish casting Repel, and didn't get pushed back until 5 seconds later. That's the server being overloaded and every game has that problem when traffic is way too high. The lag people tag about is persistent. Do the running Esuna or mount chocobo test (autorun on, just keep casting the said spell) at any time you want. You'll notice the spell always casts at least 50% no matter when you tried this, so it takes about 0.5s before your client realizes you can't cast a spell while running even though the action is actually impossible. That's the part that is unique to this game.