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

[Gregory Heytings]

> Regarding the long-standing bug reports, we did solve a bunch of issues 
> already. One major one, IIUC, was redisplay of already fontified text on 
> long lines.

Try to open the dictionary.json with Emacs on master a month ago.  It's a 
small file (only 18 MB).  On my computer just opening the file with emacs 
-Q takes 220 seconds.  220 seconds during which Emacs is completely 
locked, because of font-lock mode.  If you're not convinced, turn 
font-lock mode off, open the file, and turn font-lock mode on.

> Another piece of the puzzle was added by Stefan in 15b2138719b340.

That looked promising, but sadly it had only a very limited effect.

> So perhaps we should re-evaluate the testing scenario to see where the 
> current bottlenecks are. If we current main issue is the 55s spent in 
> syntax-ppss, a more constructive approach would be to look into 
> optimizing parse-partial-sexp. Or even give up on certain scenarios, 
> admitting that waiting 55s once to visit the end of a 1 GB buffer is not 
> so bad (and that could part could also be sped up by setting 
> syntax-propertize-function to nil and using a very simple syntax table, 
> for instance).

It is bad, especially now that it became clear that in fact it's not 
"waiting 55s once" but "waiting 55s each time the buffer is modified and 
you move to another position in the buffer".