Slow startup and operation from cloned drive

[justal]

If I try to boot from an external drive containing a cloned copy of my internal HD it takes forever.

The start-up takes a good 5 minutes or so and once it has started up the system is very unresponsive. Opening apps takes ages and the system is pretty much unusable. Shutting it down takes another 5-1 minutes as well. I'm assuming this isn't normal behaviour? What could be the cause?

This is a new 2020 i9 iMac running the latest version of Catalina 10.15.6 and the latest version of SuperDuper! to create the clone. The clone is on an external drive connected via USB 3. I've actually got two external drives, a WD and a Lacie but booting from a clone on either of them is equally as painful.

About this Mac > System Report... > USB seems to suggest that the connection to the drives is fast enough.



Has anyone got any ideas why this would be?
Thanks,
Alan.

[dnanian]

First boot on spinning drives under 10.15 can be quite slow. Booting from it again should be significantly faster (don't update the backup in between).