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

[Gregory Heytings]

> > 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.

It is, indeed.  But here we are talking about big files (say 1 GB).  In 
that case (and in that case only) it is much better, from a user 
viewpoint, to wait say 20 seconds before the file is opened and being at 
that point able to freely move through the file, instead of waiting only 6 
seconds, and then having to wait another 10 seconds between two motion 
commands like M-> C-p.  In fact, no user expects that a 1 GB file would 
open instantaneously.