Although JS supports prototype mutations, the with operator and other constructs that make optimization harder, typical JS code does not use that. Thus JIT can

Although JS supports prototype mutations, the with operator and other constructs that make optimization harder, typical JS code does not use that. Thus JIT can add few checks for presence of problematic constructions to direct it to a slow path while optimizing not particularly big set of common patterns. And then the JS JIT does not need to care much about calling arbitrary native code as the browser internals can be adjusted/refactored to tune to JIT needs. With Python that does not work. There are simply more optimization-unfriendly constructs and popular libraries use those. And Python calls arbitrary C libraries with fixed ABI. So optimizing Python is inherently more difficult.