It's not unusual to have 100% HDD activity if you have extensive startup processes, and most people have far more than they think they do.
But, if I were in the situation Kilroy describes (not usable for about 20 minutes) I'd be over my IT department like white on rice. There's something wrong if a boot up takes that long, regardless of the drive type. When I have times like that it's usually because the system is trying, again and again, to start something that either has been removed or keeps failing to start and it keeps retrying.
My own system can take between one and three minutes to become completely responsive to me after a Restart or a complete shut down (as in Fast Startup is not enabled) and starting it up again.
Brian AKA Bri the Tech Guy (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall