I guess I've answered my own question. I poked around some more and found a years-old reference to a refusal to boot being associated with a peripheral which had been mounted previously, but which was not present at boot time. I remembered that I had attached then disconnected an Apple SuperDrive, and I noticed a heckload (viz 35) of these entries:
vcpu-0| I120: CDROM: Emulate GET CONFIGURATION RT 2 starting feature <xx>
where xx ranged from zero to 65. Grasping at straws, I
- booted the VM
- physically attached the SuperDrive
- used Virtual Machine > USB & Bluetooth to dismount and remount the SuperDrive a few times
- [got distracted playing around with the toolstrip to do the same thing]
- left the SuperDrive mounted
- shut down the VM
- booted the VM. It was still kind of slow, but not as slow as before
- dismounted the SuperDrive via Virtual Machine > USB & Bluetooth
- physically disattached the SuperDrive from the laptop
- shut down the VM
- booted the VM. Voilà, boot time now back to normal