You need to select specific mirrors manually, or select based on region, but pacman-mirrors is not meant to benchmark mirrors really, it can kinda sort them based on a quick metric (ping, download speed of small file) but if the mirror you have selected isn’t fast enough remove it manually. Mirrors should all be on dedicated machines with high speed connections, but some can have low bandwidth so depending on what you consider slow it might not be for you.
I recommend to use Global CDN servers.
sudo pacman-mirrors -c Global
sudo pacman -Syyu