It perhaps depends a bit on the scene, but for instance my old Raft from Raft on Sea in Rhino 7 renders 1500 samples at 1600x600 resolution in 4 minute 7. On Rhino 8 it took a little under 15 seconds - granted, there are some errors still that need to be fixed, but it is quite a bit faster.
Time to get first pixel on the screen is also considerably faster since Rhino Render doesn’t do texture baking anymore for procedurals, those are now natively supported. Additionally image loading is blazingly fast for the renderer.