Using or not HidePrefix does not have any impact in performance.
However, if allows developers to hide those fields that designers will not need in their charts, think of key fields or composite fields created to link tables, but which you don't use in any of your expression, as they will not appear in the dropdown lists in the expression editor or the "Select Fields" menu, unless the Show System Fields checkbox is ticked.
Hide Prefix is just to make the fields available but can be hidden from expressions and dimension until we have checked option show System Fields.
Hidden field are helpful for matching and debugging.
Many time these fields are used for calculated dimensions in charts and tables,to hide multiple selections by selecting a dimension.