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?
You are right, I actually needed the flush-lines command instead of
kill-matching-lines (which is almost instantaneous).
Sorry for the noise, you can close the report.