It seems optimizing my catalog was quicker than in former versions. I clicked the button and went to prepare a cup of tea, even before I returned from the kitchen, optimizing had finished. Great!
Hopefully the black side panels no longer show up. They hadn't appeared for a while, but suddenly returned (daily!) with the last LR CC version. I'll keep my fingers crossed.
Creating thumbnails of my X-T1 files seems to take longer than of X100T files - no problem, just an observation. I just returned from Italy and imported files from both cameras, noticing the difference. But maybe I'm still tired from the long drive.