bug#25706: 26.0.50; Slow C file fontification

[Mattias Engdegård]

>> https://raw.githubusercontent.com/qca/qcamain_open_hal_public/master/hal/ar9300/osprey_reg_map_macro.h
> 
> I tried reproducing this on a pretty new laptop, and opening the file in
> question (with your settings) took less than a second with Emacs 28.

My lappy is less new but not really that slow -- compared to the hardware of 
the original reporter it's a speed demon --
but opening the file takes almost 4 s here. More importantly, scrolling through 
the file is painfully slow.

The code in the file is nothing out of the ordinary; it consists of macros that 
are 1-3 lines each; definitely not a pathological case. The entire 
fontification takes 64 s for this file.

I'd say the complaint is warranted, even if the original reporter is no longer 
reachable. Reopen?

Alan, do you have a diagnose?