bug#68006: 30.0.50; Image-mode speed

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 24 Dec 2023 17:44:37 +0100
> From:  Manuel Giraud via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> I was looking into performance issue using image-mode when going from
> one image to the next/previous (with 'image-next/previous-file') and I
> found those lines in "image-mode.el":
> 
> --8<---------------cut here---------------start------------->8---
>     ;; Discard any stale image data before looking it up again.
>     (image-flush image)
>     (setq image (append image (image-transform-properties image)))
>     (setq props
> 	  `(display ,image
> 		    ;; intangible ,image
> 		    rear-nonsticky (display) ;; intangible
> 		    read-only t front-sticky (read-only)))
> --8<---------------cut here---------------end--------------->8---
> 
> The call to 'image-flush' defeats the entire purpose of having an image
> cache.  If I remove it, moving from previous/next is much more faster
> (once this image is in the cache, of course).  Do you know why this code
> is like that?

Look at its Git history, and look up discussions and bugs from those
times.

AFAIR, one problem is that images can change behind our backs.  But
maybe there are other reasons.