Automobilista 2 is one of the best running VR titles, but even there I need to cut several settings, and keep supersampling at modest level.

I'd argue that GPU's aren't ready yet for VR. Maybe 2080 ti is. 1080 ti certainly leaves a lot to be desired, on Oculus Rift S.

I mean, have you tried other sims in VR? In my experience, they all are heavy, AC doesn't really stand out in a negative way among them. Even with 1080 ti, it's always a compromise of bad settings and blurry vision (can't afford lot of supersampling), in RaceRoom, rFactor 2, pCars 2, Dirt Rally 2 and Automobilista 2 also. ACC runs the worst, but AC runs actually quite well.

The reason other sims use simplified physics for AI, is precisely CPU stress and FPS performance. Kunos is the only studio, that uses real physics for AI, same thing makes ACC demanding on CPU too.

Well, VR in ACC is pretty garbage, the rest of the sim is phenomenal. VR is better here at least, one of the best VR games out there actually.

As for the CPU thing, having AI cars that actually follows laws of physics is pretty neat to have. This is in no other sim (other than ACC).

Means, you can copy their driving lines, and their car behaves like yours does. In so many sims (rFactor 2, Automobilista, pCars 2 etc) you can notice how AI has super grip or more top speed than you, and they can tackle corners in a way, that your car never can (despite being the exact same car). It's because they have simplified tyre and suspension physics (rFactor 2 and all ISI motor based games) or literal super grip.

But yeah, it has CPU cost, because the calculations for player car alone are heavy and when you multiply that by 20...

As for your CPU comment, that's what I would call poor optimization. You wouldn't? Sure, "terrible" might be a bit of an overstatement, and as a hotlap simulator, I'm all in on AC, it's a fantastic piece of software regardless.

I called my system upper-mid. It's certainly not high end by any means, but if a potato can render 100fps, I'd expect my specs to be able to do more than 45-60FPS (with no AA and no post-processing; I basically set everything down as low as possible to see what was happening, and it didn't do much.)

As for the VR vs. flat argument, while I understand where you're coming from, this wasn't released as an experimental feature, it's part and parcel of the game. If it's in the game, I believe it's legitimate to expect it to function at a decent level.

You can use the peformance graph UI element to see if it's CPU or GPU bottlenecking. Adding lot of AI adds CPU stress, because they use same physics calculations as player (unlike most sims, which use simplified physics for AI), very demanding for CPU

Amount of AI is CPU stress, although loading lot of different cars requires VRAM too. But indeed sounds like something with your system, I could handle full grids even with my old 4770k (currently 8700k)

VR is demanding though, always has been. Not just in Assetto Corsa. 1070 isn't that powerful card, when talking VR. Even my 1080 Ti (older, but stronger card) struggles in some games, and I have to lower settings in all driving sims, to make VR happen. Also depends on your headset (more pixels = more demanding), your supersampling settings (very demanding) etc. which you didn't mention

Most people still play this on flat screen, so calling the optimization terrible is a bit misleading

Sounds like some things wrong whit you system.. On my budget gamer pc Ryzen 2600 16gb ram 1660super I have capped fps at 100fps and whit 20 cars at the same time never drop even 1 fps and that is whit all grafics on max and allso aa..