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

[Gerd Möllmann]

Eli Zaretskii <eliz@gnu.org> writes:

>> My bet is indeed on the mere presence of text properties, plus the
>> fact that we need to merge faces.  But I could well be wrong.

Can't say something about face merging, but "frequent" changes of faces
certainly have an effect on iterator performance.  It stops, looks up
properties again to determine the next stop pos, does what has to be
done for current properties...

> Btw, I think the best tool for determining this is run-time profiling,
> such as with perf on GNU/Linux.

Yes, I don't think there is something comparable on macOS.  Or I simply
can't find it.