Camera.RenderToCubemap super slow at 8k per face, even with just 1 face

I'm using Camera.RenderToCubemap, and I had been using a resolution per face of 4096x4096. I generally render just 1 or two faces of the cubemap at a time, and at that resolution my app got 90+ frames per second. When I switched to 8192x9182 per face, I expected a frame drop, but damn - it dropped to 2 frames per second. 2! I'm using just one fragment shader (using replacement shaders), it's super simple, it applies the same color to all fragments in an object (color value applied to the materials as the objects are spawned), no lighting, nothing.