I really love this game but it's unplayable on Java, I did think it was my PC that was bad, but then I did try with a high-end friend's PC and it was the same, lots of lag, audio skips, and stuff you know.
So, why don't you make the game on Unity or Unreal Engine? They are even free if you don't gain 100.000$ a year from the game (or at least, Unity is)... and since you have already all the assets you won't have to start from zero.
Java is not meant for videogames, so please, stop using it for videogames, the results are terrible (remember when Minecraft was on Java?)