I'm having an issue with Lightroom CC being very slow at file handling. In Lightroom 5 I could move files from one folder to another on the same drive very quickly (almost instantly, never more than a few seconds), but in Lightroom CC I'm currently moving 400 CR2 files to a different folder and it's taken 10 minutes. This is on a Retina MacBook Pro with an SSD.
Also, the way Lightroom CC converts to DNG seems to be different. Normally when I import files I choose to convert to DNG in the Import dialogue. Normally in Lightroom 5 I ended up with a folder full of DNG files. However, in Lightroom CC I seem to get a folder of CR2 files which are then converted to DNG. I tried moving these files (from within Lightroom) while this process was going on and Lightroom got very confused and around 25% of my files could no longer be found by Lightroom (they were still on the disk, but with "DNG" extensions as opposed to the "CR2" which Lightroom seemed to be expecting, even though it had just converted them!).
Overall, this has slowed my import workflow down hugely. I used to Import as DNG, move my files around and boom, done. Now I need to import as CR2, then move the files into the right folder (which takes at least 100x longer now), and then convert to DNG. Not good!