Your example looks indeed very nice! But what I'm doing is entirely different. Each component is formed of the same Block component. They're not separate but rather all part of the same ListView. Each block can be dragged and dropped anywhere. The Kanban tasks support drag and drop. The entire editor support from scratch Undo/Redo, copy/paste, Markdown formatting (upon changing cursor position), multi-block selection and edit (quite a difficult problem, especially with rendered HTML, and Markdown/Plaintext underlying), and much, much more. All of that while loading War and Peace under 0.78 seconds on a 2016 Macbook Air.
I made a video to showcase it a little bit better: https://www.loom.com/share/79bbfce7feb44631a3ddfb1f229bf141
Still, Slint looks very promising! I might even convert to it in the future when it matures.