How you manage disk storage affects performance in four ways:
How you divide a limited amount of storage between indexes and data, among table spaces, and among buffer pools, determines to a large degree how each will perform in different situations.
Wasted storage in itself may not affect the performance of the system that is using it, but it may represent a resource that could be used to improve performance elsewhere.
How well you balance the demand for disk I/O across several disk storage devices, and controllers can affect how fast the database manager can retrieve information from disks.
Reaching the limit of available storage can degrade overall performance.