bug#57669: 29.0.50; C-n, C-p off under long lines

[Eli Zaretskii]

> From: dick <dick.r.chiang@gmail.com>
> Cc: gregory@heytings.org,  57669@debbugs.gnu.org
> Date: Fri, 09 Sep 2022 09:08:46 -0400
> 
> After all Commercial Emacs's original attempt at "making the best of
> a bad situation" was the impetus for your own poke at the pig.

This has NIH written all over it.

> And to be clear, you're going from "ineffable" performance to just
> "underwhelming."  As of cb036a7 you still need find-file-literally for
> some semblance of respectability.
> 
> OUTPUT:
> 
> Under long-line-threshold=nil find-file 62s
> Under long-line-threshold=10000 find-file 27s
> Under long-line-threshold=nil find-file-literally 30s
> Under long-line-threshold=10000 find-file-literally 4s

It should be clear that comparing performance for 10000-character
lines with performance for 10001-character lines will produce only
mild speedups (but two-fold speedup is not to be ignored).  In the
wild we see files with much longer lines, like tens of MBs.  So if you
want to be fair, measure the performance when lines are much longer
than the threshold.  For even more insight, see what happens to
performance as function of line length.