Best Way To Improve Davinci Resolve 12 Playback Speed?
Try generating optimized media (i.e. proxy files). Optimized Media is a nice feature that will allow you to playback, edit and grade video files that are optimized for smooth playback, but Resolve will still render from the original video files when you are ready to deliver.

Head over the Media tab and right click on the file(s) you want to optimize, and select "Generate Optimized Media". This may take a little while to run, so grab a coffee while you wait. 

You can choose what compression format to use as optimized media (ProRes, DNxHD, etc) by opening Project Settings > General Options. 

Here's a nice video tutorial that will walk you through it in more detail: