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

[Dmitry Gutov]

On 05.08.2022 16:17, Stefan Monnier wrote:
> 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.

10000 seems too low, but if the 2 seconds in the dictionary.json example 
feels too much to people (and the file is 18 MB), maybe restrict syntax 
highlighting to the first 1 MB of each file? At least until someone 
optimizes parse-partial-sexp to work much faster.

Or 10MB. Not too important as long as the value is separately customizable.

Anyway, I think I'd prefer no highlighting at the end of those large 
files, rather than arbitrarily incorrect one.