Thanks for tracking this down. Since you posted this more than a year ago I thought that the problem might have been fixed in newer versions of the open xr plugin but Im seeing a fixed 13 milliseconds on the game thread with only a few things running in game in the latest unreal 5.4 release.
However, because Im using a metaquest headset I thought I would try the metaquestXR plugin for 5.4. This plugin doesn’t seem to have the problem, and the game thread immediately dropped to 8 milliseconds no longer bottlenecking the frame.
Im sure that Im going to go over 13 milliseconds at some point but at least thats no longer the baseline. Im wondering if Epic can do anything about the openxr version of the plugin?