bug#47494: 28.0.50; describe-mode is now slow

[Eli Zaretskii]

merge 47494 45379
thanks

> Date: Tue, 30 Mar 2021 02:13:09 -0400
> From: Mark Oteiza <mvoteiza@udel.edu>
> 
> >From -Q: C-h m
> 
> Takes about a second here.  Looking at the profiler, it's split between
> some new function and the garbage collector:
> 
>          118  52% - #<compiled 0x97675bce2c2c377>
>          118  52%  - mapatoms
>          118  52%   + help-fns--list-local-commands
>           92  41%   Automatic GC
>           <snip>
> 
> https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=98e3ee27472

This is a known bug#45379, we are trying to solve it.

Thanks.