"Mystic Symbol - A mysterious emblem. Reduces the spell cast speed by 50%."
If you equip Arche with the Mystic Symbol, suddenly she will be able to cast spells at record speed. It says that it slows spell casting speed by 50%, but I find that it makes Arche (I equipped it to her) cast spells faster.
If you keep getting annoyed at enemies because they attack your characters, causing them to stop powering up for the attack/magic they were going to perform, as I do, this is an extremely useful action.