Yeah, I think a lot of this can be attributed to institutional and infrastructural inertia, abstraction debt, second+-order ignorance, and narrowing of specialty. People now building these things are probably good enough at React etc. to do stuff that needs to be done with it almost anywhere, but their focus needs to be ML.
The people that could make terminal stuff super fast at low level are retired on an island, dead, or don't have the other specialties required by companies like this, and users don't care as much about 16.7ms on a terminal when the thing is building their app 10x faster so the trade off is obvious.
Interestingly (or possibly not), since my very first computers had ~4K of RAM, I became adept at optimizations of all kinds, which came in handy for my first job - coding 360 mainframe assembly. There, we wouldn't be able to implement our changes if our terminal applications (accessing DB2/IMS) responded in anything greater than 1s. Then, the entire system was replaced with a cloud solution where ~30s of delay was acceptable.
I think the Internet made 'waiting' for a response completely normalized for many applications. Before then, users flew through screens using muscle memory. Now, when I see how much mouse clicking goes on at service counters, I always think back to those ultra-fast response time standards. I still see a few AS/400 or mainframe terminal windows running 'in the wild' and wonder what new employees think about those systems.