The ListSerializer deserializes a list as ArrayList with exactly the required capacity, i.e., number of serialized objects.
Several operators in the Table API have a MapState<Long, List<X>> to store received elements in a list per timestamp. Hence, retrieving the list and adding one element to the list is a very common operation.
Since the list which is deserialized has no room left for adding elements, the first insertion into the list will result in growing the ArrayList which is expensive.
I propose to initialize the ArrayList returned by the ListSerializer with numberOfSerializedElements + 1. This will only marginally increase the size of the list and allow for one insertion without growing the list.