This 300ms polling is a game-breaking issue for me, and I really hope they do fix it somehow; no client prediction is also very bad considering all the existing titles using it, and therefore theoretical methods available.

My guess is that a much, much lower interval would at least be a solution (30ms max would be acceptable, I guess); client prediction (by interpolation or extrapolation, I don't know) should be considered too. Most AAA MMORPG do this, and it's just expected for an enjoyable and actually challenging PvE/PvP. Right now it's frustrating, and since I don't do "rage" in games, it just makes me laugh at how badly this netcode was designed everytime I get hit by an AoE that I had seen myself escaping on the client.