AMD has released a brand new driver for its graphics cards. According to the release notes, the AMD Radeon Software Adrenalin Edition 18.9.3 driver improves performance in Assassins Creed Odyssey and is optimized for Forza Horizon 4.
Source: N4G PC AMD Radeon Adrenalin 18.9.3 is optimized for Assassin's Creed Odyssey and Forza Horizon 4