I think the problem with slow user interfaces is that the programmers who knows how to make things fast usually want to work on more interesting algorithms instead of optimising UI interactions. I built model evaluators at Google so I used a lot of these algorithms, specifically topological sort is useful in so many places and you can't really make a generic solution for it to put in a library, and different parsing methods. When doing this I had to look at how many nanoseconds different ways to do things costs, because things are run billions of times on large models in production so it is very expensive.

The people on that team were really good at making things run fast, but I haven't seen that kind of people doing user interfaces, instead even at Google some internal pages took minutes to load with barely any data, I could run a batch job spinning up thousands of servers over terabytes of data in the time it took just to view my account in that service. The silver lining is that Google has many alternatives for everything so I could use something else instead.

I guess a part of it is that Google pays for server compute, but not client compute, so your computer running slowly isn't a big deal compared to their server costs, so the optimizing programmers gets placed on backends. I did work on message routers as well, they have to be blazing fast, so I know how to make network requests fasts, the only reason the UI's are slow is that the companies aren't prioritizing it.