I do have a pretty good iMac (Retina 5k, Late 2014, 4GHz Intel Core i7, 32 GB RAM with a 3 TB Fusion Drive) running the latest stable macOS Sierra. When powering on, the log on screen appears pretty fast. But when I entered my credentials, the mac takes about 10 minutes until it's usable. I can click on windows before, they react after a long time, but I hear the hard drive for ~ 10 minutes and only after that the mac is really usable.
What could be the cause for this? I'm sure this isn't normal as my MacBook Pro is much, much faster (although there's an SSD, not a fusion drive).
What could I do to find the cause? Maybe the Fusion Drive is messed up?
I just found that disk0s2 is my rotational disk, disk1s2 is my SSD, I found screenshots of the storage system report which show it exactly the other way round... may that be the problem?