The same issue appeared long back to October when the first big update aired. It was fixed by November 20th update but reappeared with the Fortune Island update.
In short word, the game cannot load the map content when driving fast and the CPU usage would drop seriously at the same time. You may also notice a significant peak of disk (D:) usage when the FPS drop appears.
See here: https://i.imgur.com/Rtn3ZGZ.png
The problem is regardless of GPU or memory since turn all of the graphical settings to the lowest do not make any difference and my game was running perfectly when it first launched in October.
Further information and people’s reports could be found from the first post: https://forums.forza.net/turn10_postst137008_Terrible-fps-drops-after-update--video.aspx
PC build:
Quad-core I5-8300U overclocked to 3.91 Ghz
Nvidia GTX1060 MAX-Q 6G
8G DDR4
I can assure you that my PC is well functioning. I bought it only a few months ago and it’s running a bunch of other games perfectly.