I write in languages with and without IDE. With autocompletion, I'm more productive than without. Even in shell I prefer to use autocompletion or two panel file commander, because it lowers burden on my brain. I don't need to spend brain cycles on correctness of typed code. I know names, I type fast, but with Tab it even faster to type, and less error prone.

For example, when I started to use Rust, IDE was not available, so I used plain text editor for about 2 years. When I switched to halfbaked IDE, my productivity is raised because of autocompletion and error checking on the fly.