I've been feeling very comfortable with my Vim + Tmux setup recently. Navigating around shells and files isn't taking much mental effort anymore. Wohoo!
It's taken about 2 years of working full time with vim to get to the stage where the commands are so intuitive that:
I pepper text files outside of vim (email, notes, etc) with vim keys accidentally -
I can't remember what the command is to do something if I'm not actually doing it. When I need to do an action, I do it from muscle memory and I only pay attention to the underlying key press if something goes wrong.
This is noticeable when trying to find an unbound key combination for some new action, or when reading an article about vim and thinking "that's new" when actually I've been doing it without noticing.
A pleasant surprise has been that it doesn't take much effort to rebind a single command and retrain myself to use it. This is presumably because the mental effort for all the other commands has become negligible. In the early days, retraining a key combination took a lot more effort because I was already making an effort to get used to doing things in Vim.
I can work even when my vision is blurry (and my speech slurred and my head heavy) because I can use text objects and navigation commands to get to where I know text is. I'm not saying I should work when I'm that tired, but I can, if I'm already familiar with the code/file..