I mean she probably can kill All Site admin with that speed.
But she didn't know what kind of abilities they have. If you want to risk to kill them by using all life span, it will be a gamble.
If one of admin have some kind of barrier attack on them. She will be instantly killed and followed by the rest of them with no one to help.
(Honestly, Aya should be the one who teleport the other OR use the speed with teleport magic together. But, the time is too short to make a reasonable action. It's fair, I think)
By saving them, it may be better choice. Because of those reason.
I just try to reason with manga a bit. Hope you don't mind.