After you create a blend shape deformation, you can delete the target objects to free up memory and improve the performance of Maya. When you delete a target object, the blend shape deformer keeps the target deformations in memory and the target shape's weight slider deforms the base shape as if the target object remained.
You save the most memory when you have complex target objects that have only a few components that have moved slightly from the base shape. For complex targets that have many components moved from the base, you save the least memory.
You can delete the target objects manually from your scene, or you can have Maya delete the target objects when you create the blend shape deformer on the base object.
To delete a target object after creating a blend shape deformation
To delete the target objects when creating a blend shape deformer