to quote Toms Hardware
"If there was any doubt to the GeForce RTX 3090's performance, Nvidia just confirmed it via a blog post today. The GeForce RTX 3090, which Nvidia is dubbing as the big ferocious GPU or "BFGPU," is only 10% to 15% faster than the GeForce RTX 3080 at 4K gaming."
And those are nvidia's numbers, so in reality probably less.