Purchased a replacement 27" iMac in November 2017. For the first two months it zipped through everything I did, as would be expected of a "clean" device. In early January temporary screen freezes started, at first 1-3 times/day, then more recently 6-10 times per day. The temporary screen freezes generally lasted 20-30 seconds. When this happened the page I wanted to access would eventually load, but the uncertainty of when it would happen and the delay it caused was maddening. When I thought back to when the problem first arose, it suddenly dawned on me that it was in early January, shortly after I began using the new Adobe Lightroom CC, which involved syncing photos to the cloud. Oddly, screen freezes never occurred when I was actually using Lightroom, only when I was using Safari or Google Chrome to access various internet sites.
Though I haven't been using Lightroom CC heavily, it was the only significant change in my iMac use since acquiring the new hardware. As an experiment, I went in to the Adobe Creative Cloud app and turned off Lightroom CC's syncing feature. Voila! First day it was turned off, ZERO temporary screen freezes. It's easy enough to turn that feature back on when I use Lightroom CC, and off again in between those sessions.
