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

[Gregory Heytings]

> > What I'm not sure of is how useful is a "font-lock with arbitrary 
> > narrowing", where portions will be highlighted as strings rather than 
> > code (and vice-versa).  I don't have enough experience with it yet to 
> > be sure.  Taking a step back, I suspect that the only "real" solution 
> > is something like `jit-lock-defer` coupled with a way to perform the 
> > font-lock (and syntax-ppss/propertize) in the background.
>
> IIRC I heard that "some other editors" take the approach of restricting 
> syntax-highlighting to just the beginning of a large file.

Other editors just give up syntax highlighting altogether even with the 18 
MB file (and you cannot edit really large files with them).

But if you're so annoyed by mis-fontification, why don't you just turn 
font-lock mode off?

Also, why did you not protest vehemently when Stefan added 
syntax-wholeline-max, which also causes occasional mis-fontification?