Comment by Rayhem

If you're really accessing those fields it's overwhelmingly likely that you're accessing the same field for all of your instances. Your access pattern is going to be

    monsters[0].health = //calculation
    monsters[1].health = //calculation
    monsters[2].health = //calculation

and not

    monsters[0].health = //calculation
    monsters[1].name = //update
    monsters[2].is_poisoned = //check

Striding over your monsters array means you have to load each monster to access one field which just blows your cache to hell. Much better is to pack all of your health values together in a structures-of-arrays layout as opposed to the usual arrays-of-structures approach here. Entity-component-system architectures are a brilliant tool for managing this with some pretty performant results.