On my machine, VP17 has made a huge difference in playback speed for XAVCS material. Last project I did I was able to use 1080 and 4K footage natively (on a 1080/25 timeline) and throw some colour grading fx around, no problem.
What I noticed though when I embedded a pre-cut interview as a nested timeline underneath, the performance dropped quite significantly. Even though the actual video is not being seen for the most parts. Muting the video track fixed those issues.
Leads me to believe VP is wasting resources on computing whats not being seen anyway. Perhaps there's better ways 'ignoring' video tracks if they're 100% masked by the above, thus massively accelerating timeline playback. mm?