Here's the release notes:
There are two issues impacting the usability of Windows Mixed Reality on this build. Windows Mixed Reality runs at a very low frame rate (8-10fps) that could result in some physical discomfort. And there are multiple crashes at startup that will cause Windows Mixed Reality to not work. For Insiders who want to keep Windows Mixed Reality working – you might want to consider hitting pause on taking new Insider Preview builds until these issues are fixed. You can pause Insider Preview builds by going to Settings > Update & Security > Windows Insider Program and pressing the “Stop Insider Preview builds” button and choosing “Pause updates for a bit”.