I’m not too sure, I wasn’t part of putting out that email and the underlying message but I believe the build speed improves happened gradually earlier this year as we switched the hardware we were working with. It’s not like on the day the message went out we flipped a switch for the change.
I myself have sleep 10 scattered throughout various CircleCI configs though it’s not necessarily the right way to do it. There’s plenty of posts such as this one where people are trying to figure out the best way to wait for things. It’s something I’m thinking about and hoping to write about soon.