There is some kind of frame timing overlay in the WMR OpenXR dev tools:
https://apps.microsoft.com/store/detail/openxr-tools-for-windows-mixed-reality/9N5CVVL23QBT
Otherwise changing the OpenXR runtime to SteamVR will allow using the SteamVR frame timing features. You can quickly switch between the rumtimes with OpenXR explorer:
https://github.com/maluoi/openxr-explorer