I think the author might have a debug build enabled. If you open the console you can see timing information logged. Loading the introduction page of the docs [0] logs...

[Debug] Hologram: runtime script executed [Debug] Hologram: page script executed [Log] Hologram: page rendered in – 876 – "ms" [Debug] Hologram: connected to a server

Which is kind of crazy for a page with just text. Would be super curious to see what the perf is like on a proper prod build, it looks like they've implemented at least some part of the elixir vm in js

[0]: https://hologram.page/docs/introduction