Does anybody else have problems with go-ts-mode, especially when invoking fill-paragraph in a comment?

for performance, does it only highlight matches in the displayed part of buffers?

I don’t use use-package myself, but maybe you could write:

(use-package package
  :init
  (add-to-list 'package-archives '("MELPA" . "http://melpa.org/packages/")))

I’d expect it to understand that you are referring to package.el, where package-archives variable is defined.