I think that mental overhead is the steep learning curve that everyone talks about. I remember when I first started learning Emacs it was hard. I just had to think in this new way (I was used to cua style keystrokes), but after a month or so my muscle memory developed and I started not having to directly think about what I was doing. Once you get to that point, the overhead is gone and things become very nice.

But I'll grant you it's a hard sell to say "this is great, but you're going to be unproductive for a month". Nonetheless, I think the payout is very much worth it, and if you are interested, make sure you are prepared, and stick with it for a good long time before giving up.