Why is lexical binding so slow?

[Philipp Stephani]

Hi,

the manual states (https://www.gnu.org/software/emacs/manual/html_node/elisp/Lexical-Binding.html):

"Lexical binding opens up many more opportunities for optimization, so programs using it are likely to run faster in future Emacs versions."

However, in practice, at least some libraries are significantly slower with lexical binding, cf. https://github.com/mooz/js2-mode/issues/426#issuecomment-312506855.

Why is that? Shouldn't code compiled with lexical binding be at least as fast as code compiled with dynamic binding?

Philipp