Adding many elements to a list

[Nordlöw]

What is the most efficient way of performing lots of successive
appends of elements to the same list?

If we start with defining the list

(setq l '(a b))

then the Emacs manual says that we can use

(nconc l '(c d)))

I have noticed that this works in all cases except when l is nil.
To make this case work aswell we need to do use

(setq l (nconc l '(c d))))


Or can we use setcar or setcdr in a more clever way?

Reflections?
Per Nordlöw