The rumor comes from Bits and Chips through their official Twitter feed where it’s alleged that the Zen 2 architecture brings an average of 13% IPC (Instructions Per Clock Cycle) uplift over the Zen+ architecture. It is stated that this performance uplift is specifically in scientific tasks so maybe the numbers are from early EPYC 7nm ‘Rome’ samples which should be out in the wild. There are no gaming performance results available at the moment since the consumer samples are still away from launch.
A 10-15% IPC improvement can yield some good results considering we are also expecting to see some clock rate bumps in the next-generation processors too. That coupled with architectural changes and higher memory frequency support will end up delivering much better performance than current CPUs.