Terminal and emacs aren't there out of the box, but can be added.
A really nice and quite hackable writing/reading device...
https://blog.rot13.org/2011/12/liberate-your-kindle-3-and-ge...
https://www.kickstarter.com/projects/astrohaus/hemingwrite-a...
Edit: oops! They have a new website and a new product that fits your request more closely.
https://getfreewrite.com/products/freewrite-traveler
But that's really a typewriter. Which is somehow good if you just want text, not code. And here's the problem: these things belong to consumer segments so narrow that I don't see them getting the attention I'd like.
The tradeoff is that continually doing those faster transitions gradually accumulates errors, i.e. ghosting. Which is why e-readers tend to do a few pages of fast changes, then do a full refresh to clear things out (or before/after an image is shown). Fast-transition latency beyond something like 50-100ms from button-press to page-turn nowadays is almost exclusively due to software on the device, not the screen.