Its performance is unparalleled among cloud gaming services at the moment with the updates over the past year or so, Stadia is now a blurry, poorly upscaled mess in comparison, often locked at 30-45fps for certain titles while GFN is pushing real 4K at 120FPS with full ray tracing etc without breaking a sweat.
It will routinely outperform a local series x (nevermind a PS5) given the right conditions, but that’s subject to your ISP, location, and probably not running CP2077 at maxed out ultra RTX graphics settings just yet.