The point of a calculated field is not performance, it's keeping complexity out of multiple searches.
While you usually don't gain any performance compared to an inline eval command in your search, you also don't lose any.
By moving your case expression to a calculated field, you enable every searcher to use the result without having to know about how it was computed, without having to compute it themselves in the search, without having to update the expression in dozens of places in case something changed.
Shameless plug: For detailed thoughts on calculated fields' search performance considerations, attend my .conf2016 talk 😄
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