Org-mode allows it as well.

https://orgmode.org/worg/org-tutorials/org-spreadsheet-lisp-...

I've only dabbled with it, I am guessing for anything really substantial it would be quite slow compared to e.g. Excel.

It does. And while I like SES, it doesn't scale terribly well (in contrast to proper spreadsheet software). I did have a nice little stock dashboard using Yahoo!'s API about a decade ago paired with my purchases and such. I also made a nice little tax calculator and a few other things, I found it very useful (and used emacs lisp) for those kinds of programs. But the kind of scale that I've often seen Excel used for in businesses were a poor fit for it. Plus, good luck convincing non-technical managers (here I mean all kinds including financial managers and such) to use Emacs as a daily use tool. It's happened before, but it's not an easy sell.