The reasoning is only strange if you assume that the cause of slowness is the shared state.
CPUs have no problems with mutability. JS is slow because of dynamic typing, dispatch, flexible object layout, and an object model that requires lots and lots of heap allocation and pointer chasing.
CPUs have no problems with mutability. JS is slow because of dynamic typing, dispatch, flexible object layout, and an object model that requires lots and lots of heap allocation and pointer chasing.