Astro, prerendering a single component in a SSR page

I had a server-rendered Astro page and I was doing a quite intensive calculation on it. It ran on every page load.

So I used a partial (🆕 in Astro 3.4) and set the prerender flag on it:

---
export const prerender = true
export const partial = true
---

Then I included this partial client-side using HTMX (you could also just use a fetch() request and swap some innerHTML too).