As a newb, what editor to use for Ruby? It's one of those languages that VS Code does not cover properly and I can't really find good setup for nvim or emacs. RubyMine works wonders but I'm poor.
Also, why is Ruby "editor responsiveness" (not sure what to call it) so much worse than e.g. Python?
If you're looking for IDE-level language assistance, I can't help you, but since you mentioned nvim: I use regular vim with CoC / Conquer of Completion (vim plugin; LSP server, may not strictly be necessary for nvim), Solargraph (Ruby Gem; language server), and Rubocop (also a Gem) for linting. I previously/still use ALE (vim plugin; Asynchronous Lint Engine) because I haven't gotten CoC+Solargraph to play nice with Rubocop, probably due to something silly.
My impression with all of this running under MacVim... it's plenty responsive. It can take a while for Solargraph to index everything on startup if you're working in a big project; once it loads, it's snappy. (There's probably a way to cache that startup scan.)
I recently started using the Solargraph LSP with neovim and it's pretty nice.
Doesn't quite get you to the level of RubyMine in terms of smart-ness, but I find it's close-enough in practice. And I also highly recommend using TreeSitter for syntax highlighting if you're gonna go the Neovim route -- for whatever reason, I've found regex-based highlighters to be unbearably slow on even medium-sized ruby files.
I also work on a project that uses Rubocop for linting which is nice since it gives you some Prettier-like auto-formatting.
There may be some more goodies to be had (e.g. debugging via nvim-dap), but I haven't dug into that yet. As it stands, I'm pretty pleased with my current setup.
Also, why is Ruby "editor responsiveness" (not sure what to call it) so much worse than e.g. Python?