Thank you for the information! Now I know why.
Do you happen to know if the people on the discussion you linked to came to a conclusion about why OpenBSD relays perform not as well as Linuxes? Reading the whole thread made me believe that there is probably another bottle neck there too.
As far as I can recall no final conclusion has been made by Libertas, as to why OpenBSD performed/performs much worse on same HW when compared to i.e. Debian.
I doubt that the bottleneck is caused by that missing enable-ec_nistp_64_gcc_128 option since FreeBSD hasn't it either https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=200223
Also note that since that discussions a few new OpenBSD releases have been released.