Reproduction steps:

1. emacs -Q

2.

(progn
  (end-of-buffer)
  (insert "* Section [0/0]\n")
  (dotimes (n 150 nil) (insert "- [ ] item\n"))
  (org-mode))

3. Try C-c on an item

4. turn on (org-indent-mode) and C-c gets noticably slow(er)

Org mode version 9.1.9 (release_9.1.9-65-g5e4542 @
/usr/share/emacs/26.2/lisp/org/)
Emacs 26.2

Regards,
Dmitrii