There are a bunch of reports of something that makes the render thread block on GPU queries, and I’m guessing this might be another symptom of the same issue.
The comments in this thread suggest you might be able to mitigate the issue by turning on HZB occlusion or turning off occlusion queries altogether, although I don’t think the occlusion system is the root cause.
I’m experiencing similar issues with consistent shorter blocking regardless of the window state. It definitely doesn’t happen in older UE4 versions, so I’m guessing it’s some kind of incompatibility between certain version combinations of UE4, GPU drivers and SteamVR.