Disabling the index will make it harder to detect which items to draw without any benefit from disabling it. As the items don't move the use of an index doesn't cause any overhead.

What is important is to divide the "field" item into "subfield" and "subsubfield" items, so that indexing can show its power. The field itself is cheap to draw because you don't really draw anything but drawing all subitems instead of the set that is really visible yields a huge overhead.