Not just because of perf (though the perf aspect is annoying) but because of how often the three will get out of sync and produce bizarre results

Yeah, you are correct. But that means I have the benefit of ten years development in the web platform, as well as having hindsight on the earlier effort.

I would say the reason the perf costs feel bad there is that the abstraction was unsuccessful. Throughtput isn't all that big a deal for a parser at all if you only need to parse the parts of the code that have actually changed

All I can say for sure is that the reason the old tools were slow was not that the JS runtime is impossible to build fast tools with.

And anyway, these new tools tend to have a "perf cliff" where you get all the speed of the new tool as long as you stay away from the JS integration API sued to support the "long tail" of uses cases. Once you fall off the cliff though, you're back to the old slow-JS cost regime...