I've been using https://concepts.tophatch.com/ for sketching out UI mockups. It has a lot of awesome tools and runs pretty smooth. Nothing else can do vector as well as Concepts from what I've seen.
I love that they started over, without trying to fit old (desktop) interactions into a touch/pen interface, but I couldn't manage to get anything out of it. Even drawing simple shapes like rectangles felt convoluted.
I've been using https://concepts.tophatch.com/ for sketching out UI mockups. It has a lot of awesome tools and runs pretty smooth. Nothing else can do vector as well as Concepts from what I've seen.