CotEditor is a free and open source native MacOS plaintext editing app written in Swift. You'll probably want to change the default font to a fixed-width font like Monaco or Menlo, but otherwise it's a pretty good out of the box replacement for Sublime Text, Notepad++, etc.
Performance will be way better than Atom (which is built on the Electron framework).
Cot Editor is a free, open source, markdown compatible editor that supports custom themes. It's obviously not in the same league than Ulysses but really worth a try. Fully modern 100% swift app, performance is incredible (both ram and speed).
Maybe too minimum for you, but I've used CotEditor to write for years.
It counts and shows words, characters, and lines automatically. Besides, it has other features such as Dark mode, search and replice and so on.
It's free, so you can try to check if it's useful.