This situation is only on YouTube videos (using HTML5) instead of flash and only on this Dell 8700 system. I am now running Win 10, have the latest drivers for the AMD Radeon R9 200 card and my browsers are up to date. In IE, Edge and FF YouTube videos playback in slow frames and is not in sync with audio (which is normal). This is consistent and reproducible.
I am at a loss for where to investigate, there are no errors or apparent settings that influence how the HTML5 playback functions.