A huge portion of compute is triggered on request, so there isn't that much ability to time shift it. A build was just kicked off because I merged some code. In theory, that could happen overnight. In reality, changing the delay from 20 minutes to 12 hours would be unworkable.
On the other hand, when I commit code at 4PM on the West Coast it can be handled on a server in a place where there is low electricity cost.