In my experience, the first time that you boot a CCC clone from a HDD takes a noticeable amount of time. The second time is much faster, so powering down your backup disk may have had nothing to do with it. The next time that you update the clone, the first boot will again be slow.

Booting a clone (even the first time) from an SSD is always fast.

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365