In tramp FAQ page[1]:
    Set tramp-completion-reread-directory-timeout to nil to speed up
    completions.

Grepping this variable in Emacs codebase lists only defcustom
definition, no function refers to it. Should we delete it and update the
manual?

[1]:
https://www.gnu.org/software/emacs/manual/html_node/tramp/Frequently-Asked-Questions.html

-- 
Zhiwei Chen