On SteamVR Plugin, for HMD screen resolution that the width is smaller than height, the rendering frame rate is very low.

Development Platform & Builds XR Development
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11

You can override the function.

void FSteamVRHMD::GetWindowBounds(int32* X, int32* Y, uint32* Width, uint32* Height)
{

if (vr::IVRExtendedDisplay *VRExtDisplay = vr::VRExtendedDisplay())
{
	VRExtDisplay->GetWindowBounds(X, Y, Width, Height);
	if (*Width < *Height)
	{
		uint32 RecommendedWidth, RecommendedHeight;
		VRSystem->GetRecommendedRenderTargetSize(&RecommendedWidth, &RecommendedHeight);
		RecommendedWidth *= 2;
		*Width = RecommendedWidth;
		*Height = RecommendedHeight;
	}
}
else
{
	*X = 0;
	*Y = 0;
	*Width = WindowMirrorBoundsWidth;
	*Height = WindowMirrorBoundsHeight;
}

}