Our mirror is now HTTPS enabled, same path as HTTP ie. https://ftp.acc.umu.se/mirror/archlinux/
We do however recommend using HTTP and signatures rather than relying on HTTPS for file integrity.
While the CPU load of HTTPS on clients is negligable, the server CPU load becomes quite a task for organizations like ACC that are running on 5+ years old donated hardware. Especially when combined with the goal of being able to push multiple tens of gigabits...