I'm asking this for a customer, so maybe I'll need some time to answer if you have any questions, because I first have to ask my customer as well ;)
I upgraded his machine to OS X 10.11.3 and Lightroom CC 2015.5 with Camera Raw 9.5. He uses a Canon EOS 7D with Firmware 2.0.5
Before I upgraded the machine from OS X 10.8 everything worked fine and the images were transferred quickly. After the upgrade the transfer got slower (it takes about 5 seconds for the preview to pop up) and in the tethering-popup a small "loading-animation" appears. It takes about 10 seconds for the complete image to be displayed. This is extremely annoying for him, because he's working with models and not static objects.
Do you have any ideas which could cause that? (I already upgraded to OS X 10.11.5 and Lightroom CC 2015.6)