Horizon 3 is very single threaded which counts out fx cpu's. Their single core performance is dismal. You can't compare intel's clock speeds to amd's, 3ghz on intel is faster than 4 or 4.5ghz on amd's fx.
This tester couldn't even get the game to run 60fps steady using an i7 and a 980ti on medium settings.
http://www.dsogaming.com/pc-performance-analyses/forza-horizon-3-pc-performance-analysis/
Similar results here.
https://www.vg247.com/2016/09/27/running-forza-horizon-3-at-1080p60-on-pc-without-stuttering-is-a-challenge-for-most-systems-report/
Looks like yet another horribly optimized game released before it was a functioning finished product. Runs poorly on amd cpu's due to low single thread performance, stutters in intel when...