Here are a couple of contributing factors. First you have set the cracked section factor on the walls fairly low (0.4 in plane), reducing the stiffness. Second you have P-delta analysis on, which in Ram Frame, results in a slightly longer period. Finally, it looks like there is a significant rotational component to the mode shapes that results in a longer period than you might expect. This seems logical since the cores are centrally located and the other 2D walls are only in the Y direction.
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