I dunno if Xbox on PC can be talked in this thread, but man, the PC version of Forza is such a disappointment.
My first play was way too unstable because i had forgotten to update my drivers, but even after that, it's not smooth sailing. Races have stable framerate and rock solid frametime, but menus, photo mode and crossing the finish line are a mess. Framerate either tanks or fluctuates wildly. Not even Horizon 3 was so badly optimized.
You know what's the worst of it? I enjoy the gameplay inmensely. Driving model is way more comfortable than Forza 7, contrarily to what i expected i like the leveling system for the cars because it means that you actually have to use them, and despite being annoying at times i find interesting that clean driving is enforced during offline play, that's something that Gran Turismo should do because it's really easy to cheese single player races by ramming the CPU and taking wild shortcuts*.
*i.e. in Gran Turismo i've always, ALWAYS cheesed the corkscrew in Laguna Seca when racing agaisnt the CPU, wich earns me several seconds of advantage, here you get an instant penalty.
I want to buy at least the DLC, but wont do it until they iron the performance. In its current state the most i'm willing to do is pay for GPUltimate.