bug#42490: Emacs is very slow when navigating into a specific C++ file

[Mattias Engdegård]

Hello Olivier,

Thanks for the report! Could you try Emacs 27 (or git master), building from 
source if necessary? Those versions should be slightly faster, although the 
response time is probably well below acceptable.

If we distill the essentials of your file to some sort of benchmark, we might 
end up with:

(with-temp-buffer
  (c++-mode)
  (dotimes (_ 1000)
    (insert "OP(ed,b0) { ldir(); } /* LDIR */\n"))
  (garbage-collect)
  (let ((t0 (current-time)))
    (font-lock-ensure (point-min) (point-max))
    (time-to-seconds (time-since t0))))

Emacs 26.3 runs it in 11.9 s on this old lappy, but Emacs 27 does it in 3.3 s. 
This is a clear improvement but we should be able to do better. Alan may have a 
feeling for where the cycles are spent.