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