The same files work smoothly in CS5, but not in CC
Not in the general case. Photoshop CC is actually faster in most ways. I've measured it personally.
For YOU maybe it's not working as well, but that probably means there's something about your system that's getting in the way of you getting good performance.
It might be helpful if you would go into Photoshop CC's Help - System Info menu item, copy the results, and paste the data in a post here. There are many experts here who might be able to suggest things you can do - quite possibly without spending any money - that will bring your performance up.