bug#51602: 29.0.50; Slow completion on Windows 10 when symbol name contains unicode

[No Wayman <iarchivedmywholelife@gmail.com>]

This is a bug that was reported to straight.el's repository. 
According to the user's report, it is reproducible from emacs -Q 
without straight.el. I asked them to report it here, but they 
refused (citing a "previous unpleasant experience with 
bugs-gnu-emacs").

To reproduce:

1. emacs -Q
2. define the following command:

(defun testಠ-ಠ () 
  (interactive) nil)

3. Attempt completion

They claim completion will "hang" for some time. They also claim 
including the command in an init file will cause the init time to 
take 5 seconds longer.
User claims Emacs 26, 27.2 and current master branch are all 
affected when built on Windows 10 (Home and Enterprise using 
MSYS2).

We have a single alias in straight.el which includes ಠ-ಠ in the 
symbol name. My reading of:

https://www.gnu.org/software/emacs/manual/html_node/elisp/Symbol-Type.html

seems to indicate that unicode characters in a symbol name should 
be fine. Is that my misunderstanding?

Initial report:

https://github.com/raxod502/straight.el/issues/877

I'm unable to verify the reproduction case myself, as I don't have 
access to a Windows machine.
Apologies in advance if this is noise.