There's some discussion of time in the paper; they compare to Blender Cycles (path tracing) and at least for their <= 4k triangle scenes the neural approach is much faster. I suspect it doesn't scale as well though (they mention their attention runtime is quadratic with number of tris).

https://renderformer.github.io/pdfs/renderformer-paper.pdf

I wonder if it would be practical to use the neural approach (with simplified geometry) only for indirect lighting - use a conventional rasterizer and then glue the GI on top.