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

[Dmitry Gutov]

On 05.08.2022 16:00, Gregory Heytings wrote:
> > > 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?

From experience, I might visit a medium-to-large sized file, and I 
might search for a particular identifier inside it. Much more rarely, I 
would apply some edit inside it, in one place, and then save the buffer.

On balance, there is more likely more reading than editing involved. 
That's why I think font-lock should be assigned more importance.

In all likelihood, the file I would visit would be even smaller than 
30MB, but big enough that the majority of your improvements will be 
noticeable and welcome there. Just not the part that breaks font-lock.