Re: Is Emacs buffer scroll/redisplay slow?

[Eli Zaretskii]

> From: Fabrice Popineau <address@hidden>
> Date: Sun, 19 Jan 2020 16:34:17 +0100
> Cc: address@hidden, Emacs developers <address@hidden>
> 
> C-n or down arrow

In that case, the elapsed time depends on the auto-repeat rate of your
keyboard, not just on the Emacs processing time, right?

>  Also, did you measure the CPU time it took, out of those 35 sec?  (I
>  assume the time you measured was elapsed time, yes?)
> 
>  >  > .... and it took 45s on Windows/WSL and 35s on Windows/native.
>  > 
>  >  That is slow indeed.
>  > 
>  > Definitely.
> 
>  Not necessarily.  The elapsed time has little to do with the speed we
>  scroll.
> 
> We could expect emacs to be a little bit more reactive here.

Please repeat the experiment in a fresh "emacs -Q" right after
starting it, then, after you finish scrolling, see what is the amount
of CPU time reported by utilities such as PsList.  Alternatively, look
at the load average value displayed by another Emacs session.

In my testing, when I lean on the DOWN arrow, the system load never
raises above 0.25, which means 3/4th of the time Emacs is idle.  My
keyboard's auto-repeat rate is 30 characters per second, which means
Emacs uses up about 7 msec (1000 / 30 / 4) to process every DOWN-arrow
keypress, and that includes all the redisplay code, JIT font-lock
etc.  I'm not sure I agree to call this "slow".