That has not been my experience. JS/TS requires the most hand-holding, by far. LLMs are no doubt assumed to be good at JS due to the sheer amount of training data, but a lot of those inputs are of really poor quality, and even among the high quality inputs there isn't a whole lot of consistency in how they are written. That seems to trip up the LLMs. If anything, LLMs might finally be what breaks the JS camel's back. Although browser dominance still makes that unlikely.
> Very few people will then take the pain of optimizing it
Today's LLMs rarely take the initiative to write benchmarks, but if you ask it will and then will iterate on optimizing using the benchmark results as feedback. It works fairly well. There is a conceivable near future where LLMs or LLM tools will start doing this automatically.