Having CommonLisp as the extension language would indeed be a great improvement over Elisp.

However, I am suspicious about the dependency of Lem on Webkit, which for me is rather associated with inefficiency and low performance than with the instant reactions that I expect from programming text editors, terminal emulators and tiled window managers.

The main problem is that it is very difficult for any replacement of Emacs to offer everything that already exists for Emacs.

In my opinion, the only way in which such a replacement would succeed would be if it would include an automatic translator from Elisp to CommonLisp scripts executable by it, with equivalent functionality.