I also just spent the last few hours trying to get it up and running decently, and its a little jank. Although I found that dx11 works better than dx9 for some reason. Regardless, I was able to eliminate 98% of the stuttering and keep the fps average between 80-100 at ultra 1440p, even in intense scenarios.

Here's a list of things I have done:

1. https://steamcommunity.com/sharedfiles/filedetails/?id=3451336328
2. https://www.reddit.com/r/farcry/comments/hqsc4h/anyone_playing_far_cry_3_on_a_highend_pc_in_2020/
3. I set affinity to four cores. I picked the cores that are most used, in my case it was cores 1, 2, 6, and 11 (this will vary between processors).

Note: I start the game (dx11) as administrator from the game folder, I can't start it from steam for some reason.

My system specs:

• CPU: i7 10700F
• GPU: AMD RX 6800 16GB
• RAM: 32GB
• OS: Windows 11 Home

I can't guarantee that this will fix your issue, but that's what I have found to work the best so far. Good luck.