bug#56682: Fix the long lines font locking related slowdowns

[Eli Zaretskii]

> Date: Sun, 31 Jul 2022 22:50:23 +0000
> From: Gregory Heytings <gregory@heytings.org>
> cc: 56682@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>, 
>     Stefan Monnier <monnier@iro.umontreal.ca>
> 
> It is way better to wait a few seconds more while the file is being
> opened than to wait before two basic motion commands when the file
> is already opened.

I don't think I agree.  Having to wait for the initial display is
pretty annoying.  In fact, this was how the original font-lock worked:
it was triggered by find-file, and would fontify the entire buffer
before showing any of it.  That led very quickly to the likes of
lazy-lock and fast-lock, and was eventually fixed by jit-lock.

Try visiting a very large file encoded in something of the ISO-2022
family, and you will see what I mean.  It's an annoyance.

I have jit-lock-stealth enabled because I don't like the initial wait,
but don't want any subsequent waits, either.  We also have the (little
used, for some reason) jit-lock-defer feature: if you set
jit-lock-defer-time to a large enough value, M-> followed by C-p will
not be as slow as they are now, I think.