Have been playing Forza Horizon 4 since launch, currently playing on an Intel x3960 processor with an nVidia RTX 2070 running latest drivers. Prior to the Series 16 update yesterday very rarely saw less than 60 FPS on Ultra settings with a 1920x1080 resolution, with only an occasional dip below that. After the update the average seems to be 30FPS, with a ton of lag and audio cutting in and out. Occasionally it bumps above 45FPS, but mostly is between 20 and 30, almost unplayable or at least gives me a headache after more than 15 minutes of game time. Anyone else experiencing this? Tried messing with visual settings, dropping AA and AF, turning of VSync, etc. but has not improved things. Really frustrating.
I’m on PC and the game is still running the same as before, which is to say it runs very well, I’ve got it running at 75fps vsync to match my refresh rate. On an i7 4790K and GTX 1660. Driver version 436.48. Windows 10 Pro, 1903. Most of the settings I’m using are extreme, ultra or high.
I’d start with some performance checks on your PC.
I’m currently playing on a stock Ryzen 2600 and Vega 64 and I’m frame locked at constant 75FPS @ 1440p Ultra due to monitor refresh limit. GPU runs around 65-70% load.
The ingame benchmark gives me 120 - 130 FPS.
Your CPU should be at least, if not more capable and you have a better GPU so you should be smoking it.
Can you run 3dmark and compare to similar systems? what does MSI afterburner show for G{U load? etc.
The other PC in the house with I7 2600k & RX580 can manage ~45 FPS at 1440p and a solid 75FPS at the setting down (High?) which is twice the pixels you’re pushing.
Good luck
Thanks for the feedback. Yeah, bizarre. I also just updated to the latest GPU driver, maybe I will revert backward and see. Should be going strong with no hiccups that is for sure. Exploration continues! 
theres somthing wrong with nvidias newest drivers 440.97
And likely a GPU specific issue exclusive to Turing cards.
Appreciate the feedback. I figured it out and it had nothing to do with the update, was coincidental. My closed loop water cooling system on my CPU failed and the CPU was throttling to keep from overheating. Was at 90C. New water cooling system, now is at 39C and locked at 60FPS with only 35% GPU utilization. Thanks to all that answered!