Have you tried old UNIX editors like Vim? Or is that still too bloated?

I personally use Emacs, which is hilariously bloated with all its plugins (it even has built-in games), but it's so old that "bloated" is actually pretty fast even on my phone (using it through the command line in Termux). But, because of the bloat, I would not recommend it to you, as Vim is often seen as a less-bloated competitor. It is way better than a horribly bloated IDE, but still slower and more bloated than most alternative command line editors.

If Vim is too bloated, you could always try something crazy like ed, but ed is stereotypically really, really bad, so I would recommend doing more research to find something better. In general, command line programs, or even programs that are just primarily operated through the command line, are way faster than native GUI programs, so if you can find one that is designed to not be bloated, it will be very non-bloated indeed.

I guess you could also try using Vi (which is probably installed by default, accessible through the terminal). Vim is just an extended version of Vi, so Vi will probably be less bloated.