React-three-fiber was and still is a huge inspiration and I can’t thank Paul and the other contributors enough. Afaik they pioneered this relationship between a frontend framework and Three.js.

In essence, react-three-fiber and @threlte/core offer the same base functionality, which is to offer a declarative Three.js API for an established frontend framework. The technical approach however is fundamentally different: React itself provides an API (called a react renderer) to "transform" JSX into any tree-like data structure, whether it be a Three.js scene graph, a PDF or mails. Svelte (as of now) doesn't have that, so we had to get a bit creative and make heavy use of the Svelte Context API.

With Threlte 6 we introduced the Plugin API[0] which you may use to inject code into any component that instances an object. It's a very powerful feature that we use for many of the common abstractions in @threlte/extras such as interactivity[1] and which opens the door to more technically interesting and advanced use cases like the upcoming Threlte Studio[2].

On performance: react again works fundamentally different and therefore react-three-fiber is recommending to not use props for fast-changing object attributes[3]. Of course direct object manipulation is faster, as less computation is lost in "interpreting" a prop, but Svelte is generally faster in doing so, so we do not recommend against it – simple apps (which is the majority) totally work with fast-changing props.
Starting with Threlte 8 which will support Svelte 5, prop updates happen granularly, so we don't have to apply all props whenever a single one changes. I do want to emphasize that for more demanding apps you would use a pattern that is independent of prop update costs and no significant overhead of react-three-fiber or Threlte would apply here.

In Threlte 7 we introduced a powerful DAG-based task scheduling system[4] that not only solved long-standing flexibility bottlenecks but also is the technical foundation of many interesting future features and use cases such as fixed physics updates (that are still correctly queueing with other tasks), easily scheduling tasks before or other tasks (e.g. running things after a frame has been rendered) or making complex orchestrations of updates across the entire app.

In terms of ecosystem react-three-fiber is of course much larger, that is just the nature of react vs. any other framework. We do however offer solutions for the most crucial things like physics, animation, layout and XR. Our ecosystem is centrally organized in a mono repository[5]. By that we can ensure that changes to @threlte/core properly cascade through all dependent packages. Organisationally speaking, I put effort in making sure that Threlte is run and owned by a team right from the beginning to make sure it's long-lasting and doesn't suffer from a single sight on things.

[0] https://threlte.xyz/docs/learn/advanced/plugins

[1] https://threlte.xyz/docs/reference/extras/interactivity

[2] https://www.youtube.com/live/gkJ09joGBZ4?si=KX9ZwfWTixj6r55Y...

[3] https://docs.pmnd.rs/react-three-fiber/advanced/pitfalls

[4] https://threlte.xyz/docs/learn/basics/scheduling-tasks

[5] https://github.com/threlte/threlte