dynamiclinkmediaserver renders the video previews for you library. It does so for all videos even if you imported them a long time ago. If you want to deactivate the dynamiclinkmediaserver you can rename the executeable dynamiclinkmediaserver.exe in "%programfiles%\Adobe\Adobe Photoshop Lightroom 5.6\Support\DynamicLinkMediaServer\dynamiclinkmediaserver\1.0". LR still works. Prerenderd video previews are displayed. Video is not displayed. Change between video playback and library must be done using "g" (or grid view icon).