high sierra troubles

I clicked on the download for upgrading to OS X High Sierra Monday. Since then, the Mac has run extremely slowly. When it restarts (after an hour or more of spinning beach balls) I was able to finally access the "about this Apple" menu, and this is what it says:

10.12.6

iMac 27-inch Intel Core i5 (late 2013)

3.4 GHz

16 GB

Graphics: NVIDIA GeForceGTX...

So, perhaps the upgrade didn't actually go thru? Why then am I unable to launch any software? Every click requires a 5-15 minute wait to see if anything even happens.

I've reset the PRAM, started in "safe mode" and run the Disk Utility First Aid, all to no avail.

Now it will restart (very slowly: like 10 minutes) but I get a blank photo screen, no sign-in window, and need help. Losing billable hours here. THREE DAYS of troubleshooting and still at a standstill.