You're right, it's average wait time over all tasks. However, processing the shortest tasks first doesn't delay the long tasks much, if length is reasonably var

You're right, it's average wait time over all tasks. However, processing the shortest tasks first doesn't delay the long tasks much, if length is reasonably variable, because the short tasks are almost negligible to the long tasks, and the long take don't need to wait on even longer tasks. If long tasks are stalling indefinitely under this policy, then under FCFS the queue length with grow without bound, and that's not really much better.