There I "fixed" freebsd-update.

fix-freebsd-update.sh

#!/bin/sh

# Fork of https://gist.github.com/thefloweringash/8729473
#
# freebsd-update is a clever script that downloads a lot of bsdiff
# patches and whole files when patches are not suitable. The result of
# this process is a collection of files in
# /var/db/freebsd-update/files. If the files already exist, it will
# not fetch them again.
#
# This script provides the entire contents of a given distribution in
# the hashed/gzip'd format that freebsd-update expects, and can
# replace a 4 hour fetch with a 2 minute fetch + 3 minute expand.
#
# Run this before starting the upgrade to the new version.
# 
# Paco modified to fetch sources that are missing. I also mount the
# freebsd-update folder on my various VMs so that I only do this whole
# fetch-and-extract once, and then each VM can share this repository.
# I parameterised the source and the release version so that I can
# reuse this.

mkdir -p named

BASEDIR=${1:-${PWD}}
RELEASE="11.1-RELEASE"
SOURCE="ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/amd64/${RELEASE}/"
PARTS="base kernel doc ports"

NAMEDDIR="${BASEDIR}/named"
FILESDIR="${BASEDIR}/files"

### --- probably don't need to change anything below here --- ###

# Make some standard directories we will need
for DIR in "${NAMEDDIR}" "${FILESDIR}"
do
	if [ ! -d "${NAMEDIR}" ]
	then
		echo "==> ${BASEDIR}/named not found. Creating."
		mkdir -p "${BASEDIR}/named"
	fi
done

echo "==> Extracting all parts"
for p in $PARTS; do
	FULL_PATH="${BASEDIR}/${p}.txz"
	if [ ! -r "${FULL_PATH}" ]; then
		echo -n "Missing file ${FULL_PATH}. Fetching: "
		curl -o "${FULL_PATH}" "${SOURCE}/${p}.txz"
	fi
	echo " => $p"
	tar -C named -xf "${FULL_PATH}"
done

echo "==> supplying files to freebsd-update"
find "${NAMEDDIR}" -type f | while read f; do
	if [ -x "/sbin/sha256" ]
	then
		# we're on FreeBSD
		HASH=$(/sbin/sha256 -q "$f")
	elif [ -x "/bin/sha256sum" ]
		# I end up running this on my synology, which has a slightly different sha256sum command.
		SHASUM=$(/bin/sha256sum "$f")
		HASH="${SHASUM%% *}"
	else
		echo "Don't know how to get a sha256sum, sorry"
		exit 1
	fi
	TARGET="${FILESDIR}/${HASH}.gz"
	if [ ! -f "${TARGET}" ]; then
		gzip < "$f" > "${TARGET}"
	fi
done