Draft: Cure Tails Cloner's Hypersomnia: Up to 2 minute faster clones!!

Remove oversleeping: Saves up to 2 minutes from each installation, less from upgrades. Benchmark Result posted in: #20131

Sleep happens after all but one flush_buffers call and the duration matters much less now so I deleted those and moved sleep into the flush_buffers method.

First observed and commented on in the MR that made this possible: !1312 (comment 221986)

Closes #20131