Author of cljfx here. Cljfx is great for reactive forms. I would say JavaFX in itself is not blazingly performant, so it will hiccup on large amount of nodes, but it also has some solid virtualized views: lists and tables that can handle millions of items, while rendering only parts that are visible on screen. If you want something else, like millions of dots on a 2D-grid, you’ll probably need to bring your own culling and spatial hashing to render them efficiently.
I saw SandDance by Microsoft a while ago, that’s a library that seem to cover your use case, since it’s designed to render huge amounts of data — in the browser, using GPU.