Review the man page for securelevel(7), the following text needs some attention: The kernel does not react quickly enough when a lightly loaded server is suddenly attacked, and (2) The rtminexpire is not low enough for the kernel to survive a sustained attack. It can also be viewed here: https://www.freebsd.org/cgi/man.cgi?query=securelevel&apropos=0&sektion=0&manpath=FreeBSD+10.0-stable&arch=default&format=html
It looks like this whole section was deleted in r274362.