It takes over 20 minutes for my iMac to startup. My imac is Retina 5K, 27-inch, Late 2014 and I am using High Sierra version 10.13.6. One day it went from starting up in a few minutes, to taking 20 minutes. I have tried booting into safe mode, It makes no difference as it still takes 20 minutes. That is all I have done to this point. I was hoping for some advice on how to proceed.