Re: Observe Slowness In minibuffer-next-completion and friends

[Juri Linkov]

> This appears to be new as of this afternoon's update on Git:
>
> Type:
> C-h f next- <tab>
> Then press down-arrow to move to next completion; the completion list
> appears to be not active for about a second. I already tried setting
> completion-highlight-face to nil, but the slowness remains; from
> memory this feels new compared to yesterday

The first time I tried it was very slow, it took about 3 seconds.
Then on next attempts it was instantaneous.  This means only one thing
that you have to wait a little until asynchronous native compilation
finishes its job.

That said, I think there are more opportunities for optimization.
What I don't like is that it checks the presence of the completions
window for the every key pressed.  A possible optimization would be
to set a minibuffer-local variable when the completions window
is displayed, and unset it when the window disappears.  However,
still can't handle the case when the user closes the completions
window manually with e.g. 'C-x 0'.