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Well, you can actually reduce the growth rate, but not by that much.
The Elves have a ridiculously low growth rate.
And another question - are you referring to Duel mode or Classic mode? Duel mode shows a typical army setup. Classic mode is not yet fully functional; it's only for getting acquainted with some features. In particular, growth rates will be faster in Classic mode - additional growth rates will be awarded for upgrades, but this has not been implemented yet.