I love version 15 except it slows down after 15-30 minutes of editing. Pan/crop tool takes 5 seconds to open, and refresh rate is about 1 second while moving the crop window around. Also, when repositioning on the timeline, takes 2-3 seconds to start playing back. Restart and all is fine - for a while.
This is with AVCHD HD1080 and MP4 4k files in multicam mode on AMD 8350 and Nvidia 750ti, (with or without GPU selected), 8GB RAM.
I opened support ticket just now, but interested if anyone else is seeing this.