Clean installation? Reinstallation? Upgrade from Sierra? Update from the first developer preview?
Hard disk drive, SSHD or solid state?
Here, with APFS on an external hard disk drive on USB 2.0, attempting an update from the first developer preview: holding a stethoscope to the drive after an unreasonably long period without progression of the progress bar – the right of the bar appeared to be stuck in line with the left of the Apple – there's silence from the mechanics.
On screen, during the silence: for a short while there was a calculation of thirty minutes, then no calculation, then twenty-eight minutes … very slowly down to twenty then a leap from twenty to thirteen, and the bar is progressing, but the calculations are wildly inaccurate – APFS is excruciatingly slow with hard disk drives in some environments.