The performance of Nvidia RTX 3090 Ti is not yet known but based on the information provided by the company, the memory speed has increased 21 Gbps while AMD Radeon RX 6900 speed is 18 Gbps. It appears that the actual performance of NVidia will have a 9% jump resulting in it being a lot faster. This performance between the two GPU can be compared after the Nvidia product is available, tested, and compared with AMD.
https://www.ihqreviews.com/2022/02/25/nvidia-rtx-3090-ti-or-amd-radeon-rx-6900-xt-who-has-the-edge/