This was introduced in 2014 along with the comment (which has since
been deleted):
/* Introduce an annoying delay to stop swamping */
Modern cryptographic random number generators can ingest arbitrarily
large amounts of non-random (or even maliciously selected) input
without losing their security.
Depending on the number of "boot entropy files" present on the system,
this can speed up the boot process by up to 1 second.
Reviewed by: cem
MFC ater: 1 week
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D32984
1580afcd6e