I've been playing with an option to decrease shadow distance and cascades for interiors and dungeons. I've found that a much lower shadow distance of 25m and reducing to two cascades will cut CPU time in half for interior shadows and look almost exactly the same. This is because dungeons are mostly dark and shadows farther away aren't really defined anyway, and interiors are small environments that don't benefit from distant shadows.
But 25m feels too low outside in the sun, so the setting needs to account for custom shadow distances for both interiors and exteriors.
I'll keep playing with this and try to include this performance option by next builds.
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