The graphics are simple but it's actually a pretty process-heavy game. While I'm sure it could be more efficient it's not fair to say it's the least efficient game you've played.
For a game that "doesn't have much going on" you have a randomly generated/generating environment consisting of millions of blocks with players, monsters, lighting and relatively complex flowing water interacting with everything. Not to mention it's a Java game.
For a game that "doesn't have much going on" you have a randomly generated/generating environment consisting of millions of blocks with players, monsters, lighting and relatively complex flowing water interacting with everything. Not to mention it's a Java game.