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

[Gregory Heytings]

> > Yes.  Because as I told you your 2 seconds are 4 for me and 8 for Eli 
> > and 20 for someone else.  And that's in a relatively small file.
>
> Is it a "relatively small file" if we've effectively been unable to edit 
> such files for all of 40 years of Emacs's existence?

There are many other editors out there, isn't it?

> > Note that if it were 2/4/8/20 seconds once, and then no further 
> > slowdowns while editing the file, that would perhaps be okay.  But 
> > that's not the case, you will regularly see a similar 2/4/8/20 seconds 
> > delay.
>
> Do our users regularly edit 30MB files? And do a lot of changes in them? 
> In different areas?

You seem to think they do not.  Then why is it a problem if such files are 
mis-fontified?