I've seen it a lot, personally. I've even seen instances where the input would delay by over a second. Really wild stuff.
Can a better design help to mitigate those issues? Sure. But I don't like having to wonder whether it was my own design, or if it's something internal to the library.
I think we're talking about two different things- I've definitely seen that much delay before, but it always came down to inefficient rendering logic and/or components that re-rendered excessively often, both of which are easy to detect in profiling and usually not too hard to solve
This is the part I thought I was bizarre:
> The React profiler wasn’t able to show any latency in my render functions and using the browser profiler I saw that all time was spent somewhere inside React’s internal
There's nothing inherently slow about React internals. It does exactly the work you tell it to do. If you tell it to do too much unnecessary work, it'll happily go do that.
Svelte avoids this by magically not doing the unnecessary work. That doesn't mean it's faster, it just means your design needs to make different kinds of considerations.
Can a better design help to mitigate those issues? Sure. But I don't like having to wonder whether it was my own design, or if it's something internal to the library.