Even React isn't all that slow. If you look at benchmarks you may be impressed by how quickly it can do some operations. It's consistently a bit slower than other frameworks, but not that far off. There are (big) React apps that are quite fast, too.
So writing performant applications with these technologies isn't impossible.
Yet a lot of apps built with them are slow. The fast ones are rare.
I don't know what it is yet (happy to be pointed somewhere), but I'm fairly convinced that these technologies have common footguns that encourage programming things in slow ways. I can imagine it being related to React's coarse-grained and easy mismanage reactivity and wonder how much of an improvement Solid is in comparison (it is also functional, but is built to always recompute as little as possible rather than saying "whatever, it's cheap", which React does).