I figured out a way to do this. You can control how much of the viewport is rendered by changing the Field of View of the Scene Capture Camera.
However, this normally causes the view to be zoomed in. You can adjust for this (basically undo the zoom) by scaling your texture coordinates in the material. Here’s the formula for scaling your texture coordinates based on the field of view angle (FOV) of your scene capture camera:
tan(captureCameraFOV/2) / tan(playerCameraFOV/2)
This gives you the scale factor that you need to apply to the scene capture image in order to remove the zoom effect.
Here’s what my blueprint looks like that calculates the scale factor:
To apply this to your texture’s UV coordinates, you actually need to invert this though…
Lastly, I apply this scale factor to the UVs in my material:
It works well, and gave me a few more frames-per-second.
Another tip is to make sure the scene you’re capturing is optimized. I gained an additional 5 fps by removing unneeded dynamic lights!