Ok, we've been investigating this a fair amount. XRUpdate covers a *lot* of cases, so there's no one answer here. We added a new profiler marker to attempt to help with resolving this. The tl;dr is that almost all of the time in XRUpdate is typically spent in the Oculus runtime waiting for frame timing cadence. This is intentional. If there are situations where this does not seem to be the case, we absolutely want to hear about them. Please try out 1.7.0-preview.1 of the Oculus XR Plugin and verify that the time is spent inside the Oculus runtime waiting for frame timing.

Here are the notes I added to the bug:

The XRUpdate profiler marker covers a lot, so we're going to go over a few cases where it can spike in duration.

First of all, almost always, the biggest consumer of time in XRUpdate on Oculus platforms is going to be waiting for frame sync. This wait happens in the Oculus runtime, and they do this wait to reduce latency, maintain proper cadence, etc. In order to clarify where XRUpdate is spending its time, we have added a new profiler marker: `OculusRuntime.WaitToBeginFrame`. This is available in the Oculus XR Plugin 1.7.0-preview.1 and higher. Anyone reporting or voting on this bug, it would be great if you could try this version of the Oculus XR Plugin and verify that the time is being spent in WaitToBeginFrame. If it is, it's likely going to be related to one of the items below. If it's not, please give us feedback.

1) If you are using minimal frame time. In this case, WaitToBeginFrame will be waiting because you're running great and it's preventing the engine from feeding it frames too fast.
2) If you are missing framerate, even if barely, or seemingly right on the edge with maybe even a bit of breathing room.
3) If you are experiencing highly variable frame times. In this case you might see unusual behavior where WaitToBeginFrame is taking a long time one frame, and almost no time the next. This can happen because the runtime is trying to compensate for this variable frame timing.

For point 3, one thing we've seen is that the in-editor profiler can induce these spikes. The Unity profiler updates in the main thread, but not every frame. When it does update, you'll notice two `EditorLoop` profiler markers. The second one is the profiler updating the UI, etc. Because this is on the main thread, it causes a spike in frame time, and the Oculus runtime appears to be compensating for this, resulting in the WaitToBeginFrame timing that we see.

In Unity 2020.1+, you can run the profiler standalone, outside of the editor process. Doing this, we aren't seeing the same spikes caused by the profiler timing.

In summary, please try out the 1.7.0-preview.1 or higher Oculus XR Plugin and see if the WaitToBeginFrame marker is what is taking the most time. If so, this is in the Oculus runtime, and it is doing what is needed to maintain proper cadence and low latency.

If you don't believe that you are seeing one of the 3 cases mentioned above or something similar, please let us know.