Investigating a forum post it appears that a user was hitting between 1 to 10 fps while using playmode with his Dell MR HMD. I was able to reproduce a simular issue with the dell HMD by enabling Depth Buffer share and setting the render mode to multi pass. This caused a huge performance hit that makes the experince almost unuasble.

It appears there is a lot of time while the XR SDK is waiting for the GPU

Notes:
RS3 - 16299.248
Dell MR HMD

Reproduces in 2018.2.0a6 and 2018.1.0b11

User Forum Post:
https://forum.unity.com/threads/editor-runs-super-slow-1-fps-when-dell-visor-hmd-is-plugged-in.521550/#post-3434358

Repro Steps:
1.) Open the project
2.) In the XR settings under the Windows SDK enable Depth Buffer Sharing
3.) Open Profiler
4.) Press Play
5.) Observe the fps

Actual:
Enable Depth Buffer Sharing appears to have a huge impact on the performance