slice(stop)

slice(start, stop, [step])

• Standard indexing relies on slice(): a[start:stop:step] calls a.__getitem__(slice(start, stop, step)).
• slice() creates a shallow copy of the selected elements from the original list.
• Whenever you slice a list, the Python interpreter creates a new list and populates it with references to the elements you selected from the original list. Given that a reference needs 8 bytes on 64-bit systems, the overhead of copying references becomes noticeable when working with large lists. For example, slicing 1000 elements allocates almost 8kb worth of references.

islice(iterable, stop)

islice(iterable, start, stop, [step])

• Returns an iterator that produces the selected elements from the iterable. It skips start elements from the iterable, and then produces values until stop is reached, or until the iterator is exhausted. This affects execution speed when it needs to skip lots of elements.
• It's memory efficient because it doesn't allocate extra memory for the iterated elements, and it can be used to slice both iterables and iterators. Standard indexing (a[start:stop]) doesn't work on iterators because they don't implement __getitem__().