This is more an enchantment related on pacman (or better, to libalpm, I just release for comment a patch) but there is a trick to make it work on pacman without modification (see https://bugs.archlinux.org/task/20056?project=3)

a systemd Timer or Chron start the script:

• run pacman -Sup to get the list of all the package that need to be downloaded
• download them with pacman2pacman-get
• run pacman -Syu (eventual missing package released during download of the rest of the package may happen, so leaving pacman2pacman-get as XferCommand is suggested)

Stay updated with pacman issue to know when and if my patch to call xperfcommand in parallel will be accepted