Sierra public GM taking way too long to install

I successfully installed and use the last Sierra Developer Preview in Parallels 12.0.1 on a El Capitan PowerBook Pro host. I could not install the VM by pointing to the Sierra installer app; I had to do the `createmediainstall` to make a bootable volume on a USB 3 thumb drive. All was well.

Now, the public Sierra "GM" (Golden Master) release is out. Same deal where I could not just point the Sierra installer app at Parallels, I had to first use `createmediainstall` to make a bootable volume. I used the same USB 3 thumb drive.

Running the macOS installer is taking multiple times longer than it should. The screen with the progress bar subtitled 'macOS Sierra will be installed on the disk "Macintosh HD"' says installation will be about 7 minutes. After 20 minutes go by, it says "About 4 minutes remaining". After 30 minutes, it say "About 3 minutes remaining".



I have tried this repeatedly, always this half-hour goes by while the installer is ticking away for a 7-minute task. I have tried this with default VM settings, and I have tried this with custom VM settings such as more memory (4 gigs vs 2), more video memory (128 megs), 3 cores instead of 2.

As I said, the Developer Preview for Sierra did not have this problem. Installation went speedily and as expected.

Any ideas? Might this be a Sierra problem or a Parallels problem?