Howto theme list of nodes as item-list?
viewMultiple() also optimizes the rendering process by returning a common #pre_render callback in the build array, see the discussions and linked topics here Twig: &quot;for&quot; does not display variable content without first displaying all variable content, so you can't put the output directly in #items.

So building each node on its own is the way to go and you probably won't find any differences in performance, because both methods result in the same caching mechanism.