It's not lag, not exactly anyways. It's the way the netcode and server tick rate of the game work that makes it feel so sluggish at times. The short of it is that the backend architecture of the game isn't nearly as responsive as it needs to be for something like fast paced pvp where split second timing matters. The reason why when you use guard and it still lets through a massive amount of damage before the shield actually starts to take effect, is basically because all of those attacks technically happened before your client told the server that you want to guard, but all of the actions aren't processed until a split second later. That's the server tick rate being slow causing that.

You get used to it, eventually, sort of, but it never actually feels good since it feels like you're wading through water while playing. And this is actually an issue with the rest of the game in general, it's just not as noticeable in PVE.