Rails Deferred Content
render_async is a pretty straightforward setup that lets you defer rendering of various data blocks until after the initial page load. Sounds very similar to what HEY! is doing, allows you to set up your primary pages as static (thus cache-friendly), and defer per-account personalization until later.
Turbo Frames is the aforementioned HEY! solution, now open-source. See also this writeup on a practical application of the library.