The diffing is mostly an implementation deatil that is abstracted away. React could check to see if props.items changed instead of the nonsense it does. If it still does that in 2021.
The thing is vue uses getters and setters that would trigger direct changes without needing to diff the entire tree. It could theoretically be really fast, but it still used a virtual dom, and it was around react speeds, give or take.
I am convinced that if react really wanted to, they could make optimizations to make it really close to svelte. I remember seeing a tweet by Evan You about vue3 being pretty much as fast svelte if not slightly faster.
The point is that speed, when it comes to front-end frameworks, is a weird thing. There are other factors that are much more important like tooling, stability, code style, dealing with animations, etc.
Things like svelte's compiler approach are much more interesting and unique selling points.