@Luca959
I hope the above was helpful, and I thought of a few more (might eventually come up with a complete list).
The more recent CSPs have an in-game FPS app that tells you what processes are chewing up hardware resources (so you know exactly what to turn off, or at least turn down).
Before launching your game, deactivate every in-game app that you're not absolutely in need of while driving (also disable dev mode apps)
Disable/lower all of AC's standard visual candy (I'm sure you already have). Even shadows and reflections can be expensive.
Limit FPS to roughly 20-25% above your target (stops the PC working harder than it needs to, which also makes it run hotter).
Make sure the PC's cooling is adequate (they slow down for self-protection when they start getting hot). There are various tools for monitoring this like
CPUID HWMonitor that you can run in the background as a test, or load straight afterwards to check the current readings.
Along with the above, overclocking can sometimes be counterproductive. You get a small performance gain at the cost of a lot of heat, which in turn slows it back down if the cooling isn't amazing.
Choose a more optimised track (one that's smaller overall, or a version that has older lower-res textures, or was built with less objects).
Run in dry daylight (lights and rain can get a bit expensive too, so can grass FX for that matter).
And if all of that still wasn't enough, run fewer AI cars until you can arrange a CPU upgrade.
I wrote piece on making a budget gaming machine from an ex-business SFF PC. Probs only useful for people starting out from nothing though (also prices are Aussie $ and what I found back in April).
www.racedepartment.com
