I'm updating my 21H1 version with 'Cumulative Update Preview' KB5003214
The download has ceased 3 times, has shown download percentage at 100% 3 times and Installing percentage at 100% twice but then dropping to lower percentage and continuing.
The whole update process has been going on for 2.5 hours now,
Not streamlines at all. Very inconsistent!
I had one of the 4 computers that I updated that took 4 hrs. The other 3 took less than 1 hr. each.
Having seen some of the times taken, I have been in no rush to put myself through that, so I have been waiting.
Well I checked a little while ago and behold it was there awaiting download, so I bit the bullet and initiated the update. It got to 100% download (only once) very quick and I initiated the restart and it started doing its business and it progressed at a reasonable pace no pauses all the way through to restart.
Now waited with baited breath to see how the initialising/finalising came and went very quickly it booted into windows proper.
All in all I doubt if it took more than 4 minutes, very happy camper.