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

[Eli Zaretskii]

> 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.