bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Gregory Heytings]

> I am experiencing severe performance degradation on current master.
>
> The degradation happens when working with Org files
>
> [...]
>
> I was able to trigger the slowdown when unfolding headlines hidden using 
> 'invisible text property.
>
> Do you have any idea what could be going wrong or how to debug things 
> further?

Can you (a) provide a MRE so that we could track down what's going on 
and/or (b) try to set long-line-threshold to a larger value (20000, 50000, 
100000) and see when the performance degradation disappears?