Agreed to a degree.

I find Python in the REPL to be great, but I think Common Lisp with Slime and maybe Clojure with Cider take it to a new level. You can write your code and when it errors out it drops you into the REPL where you can update the code incrementally while it is running.

If Python gets me X% increase, Lisp and Smalltalk (and maybe Forth) get you some X% + Y%. The parent comment is right that 30% is probably a back-of-the-envelope calculation and not a rigorous one.

I'm only a novice/superficial language geek, so I can't tell you why Lisp and Smalltalk can do that though. Extreme late binding? Hopefully someone more educated can reply.