bug#74813: 31.0.50; scroll-down is 10x slower and laggy than scroll-up

[Eval Exec <execvy@gmail.com>]

Friendly Pong. I'm busy these days. I will upload a reproducible
minimal init.el tomorrow.

On Sat, Dec 28, 2024 at 6:59 PM Eli Zaretskii <eliz@gnu.org> wrote:
>
> > Cc: 74813@debbugs.gnu.org
> > Date: Thu, 12 Dec 2024 11:11:45 +0200
> > From: Eli Zaretskii <eliz@gnu.org>
> >
> > > From: Eval Exec <execvy@gmail.com>
> > > Date: Thu, 12 Dec 2024 15:59:51 +0800
> > >
> > >
> > > I opened a large rust file, about 5000 lines.
> > > scroll-up is quick and smooth, no laggy:
> > > [...]
> > >
> > > but scroll down is very laggy:
> > >
> > >         2667  87% - command-execute
> > >         2666  87%  - funcall-interactively
> > >         2666  87%   - evil-scroll-page-up
> > >         2666  87%    - scroll-down
> > >         2665  87%     - apply
> > >         2664  87%      - ad-Advice-scroll-down
> >                            ^^^^^^^^^^^^^^^^^^^^^
> > >         2663  87%       - #<primitive-function scroll-down>
> >
> > First, you have an advice on scroll-down.
> >
> > Furthermore, scrolling down exposes portions of a file that were never
> > seen before, which needs to fontify them.  If fontification is
> > expensive, it is expected that you will see some lags.  What happens
> > if you scroll down, then go back to the beginning, and then scroll
> > down again -- is scrolling down the second time much faster?
> >
> > Also, you didn't say which major mode did you use for this file.
>
> Ping!  Can you please answer my questions?