I ran into the same issue… which is not really one.
Forza Horizon 4 is trying to make your game run as best as possible due to your settings being set to [Frame Rate : UNLOCKED (VARIABLE)]
This option might cause some performance issue or put your GPU and CPU usage to it’s maximum.
My fix would be to simply reduce the frame rate option to 75FPS or lower (depending on your screen’s capacity).
Adjusting my frames to 75 did fix my problem. Hope this can help people having troubles with that.
Forgive my English,