It could be related to your system or to the cygwin build or its general environment.

Yeah, it is definitely specific to the cygwin perl. I just installed strawberry 5.38.0 and running from the same cygwin terminal as the original test, it looks fairly normal:

0.231036 read lines from disk and do RE; n=769114.
1.459296 read lines from in-memory file and do RE; n=769114.
[download]

It would be nice if someone else using cygwin could test it. I have the current cygwin version of perl which is 5.36.3.