You could always get a fresh list of 5 most recently synchronized mirrors sorted by download speed with:
reflector --latest 5 --sort rate | sudo tee /etc/pacman.d/mirrorlist
Note that the command can take some time to run.
After any alteration to your mirror list you would need to pass an extra y to pacman:
sudo pacman -Syyu
Later just:
sudo pacman -Syu
to update.