This game is highly entertaining, although I do have a few small points of criticism. An important point is that the generator makes the maps RIDICULOUSLY difficult; I'm a pretty advanced sweeper, as seen by my high scores in the hard and expert categories, yet I feel that the expert map, especially, has far too many mines to be an enjoyable experience. The 33% of mines makes for an especially annoying challenge, when compared to other expert minesweepers, most of which have 21% mines. This, coupled with the lack of "opening luck" results in severe frustration at not even being able to play. This is not just a matter of playing poorly - the game is all luck until you either die or open a large area. I have a few other issues that I will address through PM due to the character limit, but this is the most important. Please address these concerns, as it is a fantastic game otherwise.
I'm fairly sure the Extreme map only has a 25% mine ratio (64 mines in 256 squares, the 192 at the end of the level is the number of non-mine squares). And by "opening luck" I assume you mean that it is not possible to die on the first click. If so, see your below comment. The difficulty is perhaps too high for regular minesweeper, but this is the result of balancing the game to fit in the upgrades.
Hardcore means you can die on your first click. If hardcore is off, the game will act like standard minesweeper, in that, no matter where you click initially, there will not be a mine there.
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I did this with a certain methodology that has minimal lag for 95% of players... unfortunately the remaining 5% get hit with some massive lag :(.