Traffic. Holy cr@p.
I spent a good chunk of today testing this very question on the ground at KJFK.
With the in-game vsync set to 30, I turned every setting as low as it could go, but set my render scaling to 130% of 4K to get my GPU latency where I wanted it (32ms). Reduced LODs until I could pan without seeing a “mainthread limited” condition (95 on each slider, if you’re curious). Then set every setting, one at a time, from low/off to ultra, panned again several times, and took note of which settings pushed the CPU enough to drop the main thread latency below the GPU target (“mainthread limited”). None of them did.
The CPU was unaffected by everything but LOD.
As graphics settings went up, render scaling had to come down to maintain my GPU render latency target. Ended up with all graphics settings maxed, render scale at 60% of 4K, LODs at 100, 30 fps and no mainthread limited on the ground.
I then turned all of the traffic off. Boats, birds, AI…everything. I then repeated the entire process above, but ended up with all graphics settings maxed, render scale at 80% of 4K and LODs at 200. End results also 30 fps and no hint of mainthread limited on the ground.
To reiterate what myself and others have said in the past, LOD and traffic are huge loads on the CPU/main thread. Everything else falls on the GPU, and if you increase settings from low to high to ultra, reducing render scaling as a tradeoff for the increased graphics settings to maintain your target GPU latency is pretty straightforward.
I saw absolutely no appreciable impact on CPU load with ANY graphics settings outside of LOD or traffic. I tested all of them. Twice. Once with traffic on at 100%, and once again with traffic at zero.
Traffic has WAAAY more of an impact on CPU performance than I had given it credit for. The difference in LODs alone going from 100% traffic to zero traffic was +100 on both LOD sliders, which is huge.
As long as you have a target frame rate in mind, use the GPU load to set it. Do it on the ground. Reduce LOD settings and traffic density until you no longer see “mainthread limited” on your dev mode fps counter.
Hope this over-explanation helps.