How can I show vertical indentations markers with Emacs?

I guess this and this could be an answer to your question.

Installation is easy. Invoke git clone [email protected]:antonj/Highlight-Indentation-for-Emacs.git . in, say, ~/.emacs.d/highlight-indents and add

(add-to-list 'load-path "~/.emacs.d/packages/highlight-indents/")
(require 'highlight-indentation)

to your .emacs. Then, when visiting a file you can M-x highlight-indentation-mode RET to enable the minor mode.

You can also use highlight-indent-guides-mode and add it as a hook to prog-mode, so that it gets loaded automatically.

In Spacemacs, I did it by first installing it with M-x package-install or adding it as a user-defined package in .spacemacs, then M-x customize-group, prog-mode, prog-mode hooks, "ins", enter highlight-indent-guides-mode.

You can configure it via M-x customize-group highlight-indent-guides and change the indent character, coloring, etc.

So far, it works wonders and renders quickly.