Looks like it's mainly tree/code manipulation. Typing code on the keyboard is probably the least taxing thing when it comes to software development. But I guess it will be nice once it has become a "reflex" rather then a conscious key-combo.
It's not so much about reducing the amount of characters typed, and instead moving the way you think about code from the character level to a more structural level.
Calling it a "reflex" is an interesting phrase! Tools like magit let me encode complicated processes into muscle memory, in a way where retrieval doesn't have to go through remembering and typing a string. Structural editing is similar.
It's about typing code, as opposed to typing text, with all the structural, highlighting, auto-formatting, auto-completion, error-detection, etc advantages this brings.
I only started using it a few months ago. It's such a natural way to edit code, it only took me about a day for it to become reflexive.
Now it just feels vaguely annoying to work without it. It's fine, it's just one of those ergonomic changes that nags at you a bit. Kind of like the opposite of that feeling of taking off uncomfortable business clothes at the end of the day. Or what I imagine people who are better at vim than me keep talking about.
It's not just saving keystrokes. It eliminates a whole class of errors. I recently did ~4-500 lines of Clojure in CodeMirror and wanted to kill myself by the end of it.
If you know a Lisp I recommend just giving paredit a spin for a few minutes, it's an interesting experience.