It’s beyond my realm of expertise so I can’t speak confidently, but my hunch is that many of the responsiveness papercuts are cross-domain in nature, which makes them more difficult to track down and fix (not to mention makes who’s responsible for fixing them more murky).

Another big chunk likely comes down to the tradeoffs all the big DEs have made in favor of making development easier or improving DX. This is understandable but at the same time it would be nice if at least one took a hardline stance towards commiting performance reductions and placed responsiveness as a chief concern, much as the operating systems of yesteryear did.