I made a post around a month ago detailing the issues I'm having achieving smooth playback in Pr even when using proxies, and still can't find a solution. I've just updated to V15.1, created a new project, reimported my footage and generated proxies using a custom preset but still experience choppiness.
I tested the proxies on the first 3 clips and was able to edit them fine (scrub through, cut, add effects etc.) so did the same for the rest. As soon as they were finished I dragged another clip into the timeline and it immediately made the whole sequence choppy again. The proxy button is enabled and audio input is set to 'No Input' (a few things I've been told to check).