Very slow OPTIX render on 3070ti - why?

I need help. My new 3070ti card is giving me VERY SLOW render times with OPTIX. With CUDA it is performing as expected.

I recently upgraded from a 1080ti to a new 3070ti. This site ( is good for comparing rendering times using various GPUs across multiple platforms. Using this, I estimate that an OPTIX render on my 3070ti should be about four times faster than the CUDA times on my 1080ti. Instead, the 3070ti is rendering SLOWER than the 1080ti CUDA - about 80% as fast. I have no idea why.

Does anybody know why this is? As I said, with CUDA the RTX is ok - it’s about twice as fast as the GTX. Just not with OPTIX. Do I need to download additional software to get the OPTIX to work as expected? Does it matter what PCIe slot on my mobo the RTX is plugged into (currently PCIe 3.0)? It is admittedly an old board. I’ve tried both game-ready and studio drivers - same difference.

I’m using Blender 2.93.1 on the last build of Windows 10.

Thanks for any insight.