bug#59919: 28.1.50; "kill-matching-lines" takes forever

[Gregory Heytings]

> The kill-matching-lines function takes far too long. Trying to kill 
> lines matching the "ssh" regexp in the attached file takes minutes to 
> complete. The actual file I'm working on is much bigger, and emacs just 
> hangs. Reciple to reproduce:
>
> $ bunzip2 log.txt.bz2
> $ emacs -Q log.txt
> M-x kill-matching-lines
> Kill lines containing match for regexp: ssh

Are you sure you actually want to _kill_ ("cut" in modern parlance) those 
lines?  In the file you attach, all 10000 lines match "ssh", so it means 
that Emacs "cuts" each of those lines one by one, which of course takes 
(quite) some time.  If you use a regexp that does not match all lines, say 
"109.106.244.80", the effect is almost instantaneous.

Note that there is also a command "flush-lines", is that not the one you 
actually need here?