As part of the official reviewers guide sent out alongside NVIDIA RTX 2080 Ti cards, NVIDIA included a number of its own internal benchmarks across a number of titles. With 4K performance measured the number certainly sounds impressive - with Battlefield 1 showing a 48% increase in performance and Wolfenstein II 50%. In fact performance increases were in this range across a number of titles.
Hardware site VideoCardz has collated all the numbers, which you can find here
Additional titles tested include Shadow of the Tomb Raider (37% increase), Rainbow Six Siege (49% increase), and more. Naturally, as official numbers these should be viewed under the spectrum of best possible scenario - but in terms of hardware NVIDIA notes that test hardware included a Core i9-7900X 3.3 GHz CPU and Corsair 16GB DDR4. And games set to max settings at 4K resolution.
But, with the NVIDIA RTX 2080 positioned as a 4K card - it makes sense that this is where the biggest improvements would be found. Although close to 50% across the board is more than impressive.