Question VR renderViewportScale and eyeTextureResolutionScale performance

I was playing with some dynamic resolution control using XRSettings.renderViewportScale and XRSettings.eyeTextureResolutionScale. Essentially, I set eyeTextureResolutionScale=2.0 right when the app starts, and then dynamically change renderViewportScale. Everything functions properly.

However, setting a large eyeTextureResolutionScale seems to have a pretty serious impact on frame rate, even if I'm just rendering to a small part of the viewport. For example, eyeTextureResolutionScale=2.0 and renderViewportScale = 0.5 works fine, but is much, much slower than "normal."

Is this a known / understood problem? Is there something about large eye buffers that needs to be accounted for?

Thanks much!