It's not, but there seems to be a similar note taking app that is open source called AppFlowy. Silly name but it seems nice.

Seems like it misses some key features from Notion. Notion is awesome. Trilium seems cool too, but it's not enough to make me leave Notion.

This person (link below) is creating a nice reddit alt using Rust. I know its Rust because I spoke with them. I think this php site can work very efficient with proper caching, file storage (storing images and videos elsewhere), and db optimization. I think Flask/FastAPI is quite fast as well. I ran a small site using flask with 250 concurrent users using light calculations for the backend a couple years ago on my home server and it still loaded extremely fast. I am seriously curious what sort of server or shared hosting this site is being ran off of, because one article every 15min to an hour makes me believe its not extremely bogged down with traffic, yet its still not performant. Maybe its a host issue of being ran using a slow shared hosting service, idk.
https://www.reddit.com/comments/13x0hzo