I went to Forza Horizon 3 Forum and found this Moderator that you can ask about your problem and he will investigate it. :
Ill be updating the release notes soon. However, I just wanted to mention that there were no game changes made. We only added an in game message if you have an issue syncing your profile from the cloud. This would not cause you to have any of the issues you are describing here. If you have an issue, and cannot find it here, you can email me email@example.com and I can start an investigation.
so I wrote so maybe it fixes and crap it on AMD only drivers
Hello, I had same problem, Forza Horizon3 stuck on "Optimizing for your PC" (tried to restart game more than 10 times, reinstalled game, etc. every posible way and it didn't work).
I had newest amd drivers: 18.8.1
Then I found some1 posting on reddit, and I uninstalled amd drivers 18.8.1, and installed 18.3.1 version of drivers
After that Forza Horizon 3 passed "Optimizing for your PC" from first try, and I can play it again