>>> The locked narrowing currently in effect makes it impossible to 
>>> investigate this problem or improve the behavior :-(
>> 
>> How so?  It is fully backward compatible, just (setq 
>> long-line-threshold nil).  If what you want is only to remove the 
>> locked narrowing, comment out the relevant line in xdisp.c.
>
> That brings back the performance problems in redisplay, which have a 
> more pronounced effect, overshadowing both issues and potential 
> improvements in font-lock.
>

The first option does, the second does not.