It falls back to the "default" index (/oak:index/lucene), which is deprecated (but not removed), one of the reasons being that it is typically quite large.
I would not attribute the high load directly to it, this typically has other factors (e.g. because you do this query in the context of rendering a page).
if you see this message only on publish, check if the author has additional indexes which you don't have on publish.
(A side note: This query is quite silly, because it says: Give me everything below /content/dam/abc, and you can achieve that much easier with a tree traversal.)
