In Act IV, scene 1, how does the use of alliteration and onomatopoeia make the witches language faster and more rhythmic?In Act IV, Scene 1, how...

The witches say words like "mew'd" and "whined" in the opening scene as onomatopoeia.  Then they use alliteration in words like "Double, double, toil and trouble" to make use of aggressive, guttural sounds.  In all that the witches say in this scene, their words are all in either single or double syllables.  The phrasing goes quickly.  The alliteration emphasizes their gross concoction brewing in their cauldron, and the onomatopoeia makes the animal parts involved even more gory.

Ultimately, the use of these literary terms creates the wonderful imagery that goes along with the themes of light and darkness, and sickness and medicine mentioned in the link.