Thinking doesn't need words. Thinking for me is a process of ideas and concepts coming from different directions, colliding, merging, splitting, transforming, and then going into other directions to interact with other thoughts.
The translation between these concepts to words or sentences from my mouth or my fingers is effortless by the way. Which I myself find interesting. I think it should take some effort but it doesn't.
If you wonder how one can think without an inner voice then I wonder can you even think without your inner voice? And can your inner voice be faster than your vocal voice and if not, doesn't that slow down your thought process?