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

[Gregory Heytings]

> We cannot perform every basic operation in fixed time for any 
> arbitrarily sized file. There are limits of what we can possibly do.

Apparently the limits are lower than what you think.  Provided that we 
accept some compromises, such as mis-fontification, which is also what 
syntax-wholeline-max does, and against which you didn't protest.

> I'd really like it if we could scope this discussion to solving that 
> particular problem. Not the speed of operations in large files in 
> general.

I don't understand what you mean.  Which "particular problem"?  The point 
of this discussion is of course the speed of operations in large files in 
general.  If you take that out of the picture, everything is of course 
possible.  I'm not even sure what remains in fact, Emacs is an editor, not 
a displayer.