Supplementary explanation follows.
Thank you for the kind, detailed explanation!
AUCTeX has two schemes of installation.
[A] Traditional scheme using configure, make, make install
In this way, system administrator can perform system-wide installation.
I see! That means that if the system administrator elects to enable AUCTeX, all the users on that system are affected by TeX-parse-self, even if they don't use AUCTeX at all. I guess that on such a system, the parsing of the TeX directory tree happens each time emacs is launched if the system-wide default of TeX-parse-self is t. Is that correct?
I thought that AUCTeX was always an add-on enabled or installed by each user. That was my misunderstanding.
[B] Modern scheme, namely ELPA
That suggests that system-wide emacs installations with AUCTeX enabled will eventually die down. It seems to me that today, most Linux and Mac administrators use binary packages of emacs for which add-ons aren't enabled. There are cases where you want to compile emacs from source, but even then, there is little motivation for the administrators to compile emacs with AUCTeX enabled because each user can install AUCTeX with ELPA. [Actually, I sometimes compile emacs from source (through homebrew) and install it system-wide, but it has never occurred to me to enable addons.]
That would be a happier world because we don't have to worry about emacs users who don't use AUCTeX.
Thank you again.
Cheers,
Ryo