elm-sized-list implements a list that caches its size. This means SizedList.length lst runs in O(1) time, while List.length lst is an O(n) operation. All else behaves similarly.
Use this module only if you are constantly reaching for the size of a list. See the list of pros and cons to decide if you should use regular list
Pros:
Cons:
unsafeFromList to have good performance.