The problem with JS engines nowadays isn't as much about the language itself (or its core library), it's more about the absurd amount of optimization it should

The problem with JS engines nowadays isn't as much about the language itself (or its core library), it's more about the absurd amount of optimization it should have to perform well in a varied number of possible setups, and all while the browser is busy doing DOM compositing, loading yet more JS code, etc. Things like JIT are not exactly necessary if you just want your language to "run", but an important part of an optimized system. To me it seems the past ~5 years of JS engine development have been focused on what kind of optimizations to implement, rather than parsing or implement core library features.