Downloading a patch right now. Can't find patch notes.
What does it do?
It probably fixes their malfunctioning ban-hammer; I'm only half-kidding. The announcement came only 30min prior to the release of the patch as a sticky in their forums:
Today we'll be launching the latest update for Forza Horizon 3 on Windows 10. This update will feature minor updates to improve performance and stability for the Windows 10 version of the game.
Also, on reddit they claim it downgrades the game's graphical fidelity and some report 20fps gone missing or increased stuttering. Other say it's a great improvement for them. So same old, same old.