Android VR perf drop from 4.20 to 4.21 (w/ RHI Thread)

Android VR (Oculus Quest):

On moving from UE4.20 to UE4.21 with the RHI thread enabled, we are no longer running at a consistent 72 fps.

Looking at a profile capture, it seems to be because when we go to render instanced mesh particles, the render thread stalls allocating a vertex buffer (can be 10+ms). Tracking back a frame it appears that the RHI thread takes a long time to present (6ms in FinishRendering).

I tried disabling the RHI thread but perf suffered even more.

Any ideas?

I can supply a ue4stats file if needed.

How did you disable the RHI thread on Android?

How did you enable the RHI thread? I am working on it but I cannot find this thread.