You can check the boosts/requirements for a specific boss and by how much by using +monster [name]. Some boosts don't currently appear so it is advised you crosscheck against this chart.
Global Boosts
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Elysian Spirit Shield: Reduces food usage by 17.5% when equipped in the gear setup being used.
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Black Mask (i): Gives a 15% boost while killing monsters on a slayer task.
Refer to the Slayer Boosts page for more information on boosts that apply to generic slayer monsters.