Same problem here with a 1 1/2 year old iMac 21.5 Retina 4K with a 3.6 GHz i3 and 16 GB 2400MHz DDR4. It is so slow to boot up that it takes almost 4 minutes (My Windows 10 machine with a similar amount of Ram and a similar Hard Drive takes less than 1 minute). Just opening mail takes 3 minutes and same with any other application, whether Apple or third party and I am constantly looking at that spinning colour ball. Everything is so slow that I am seriously thinking of giving up and changing everything over to my windows machine. I have owned 5 Macs for the past 20 years and this is the worst operating system I have ever encountered, including on all of my windows based machines. Steve Jobs would never have let crap like this out of the building.