Resolve suddenly not able to playback smoothly

hi!
I have an interesting problem and can´t find a solution:
my Resolve cannot play anything anymore smoothly. And it happened suddenly, while working on a project.
I am working on a new Macbook pro 15 with 16GB ram and 4 GB GPU (560x).
Up to now everything was perfect, I worked on several 4K projects and all was super fast and fluid.
Today, while working on a project with mixed HD 100fps files and 4K files, it just cracked down.
I restarted everything and opened also old projects and even those are not running smoothly anymore.

Help, I have no clues what happened there

Did you update Resolve? I've noticed that with each new version, playback on my sub-spec machine (a 2013 Mac Mini) deteriorates, and 1080p timelines that used to play smoothly now are very jerky. I only use that computer as a test machine since it's way below specifications in the Resolve Configuration Guide and I expect it to perform poorly. But if you updated Resolve this might explain it.

Also check your caching settings (go to Playback > Render Cache); you should either set to smart or user; if set to user check your user settings in preferences.