Bonuses that escalate noticeably slower than enemy HP across almost all upgrade paths makes this a clear "buy store perks" cash grab. It's not really a game. It's a dressed up Skinner box. If you go that route, be careful though because even if you cough up real-world money, lots of the upgrades in the store are functionally meaningless to game progression so you can spend real money and still not get anywhere. It's not worse than lots of other idle-like games, but it's not better either.
Fun game, but it definitely slows down a lot. I know that's normal for idle games, but the curve is still pretty big. It would be nice if the offline ticks scaled to contribute more. All told, a good game though.
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Last update addressed just that, so later game paces quite a lot faster now.