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

[Eli Zaretskii <eliz@gnu.org>]

> 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?