Mystery - decent 4K performance on a GTX 1080 Ti

[atrox]

Need some expertise about some technical mystery.

Have a old but decent 1080Ti OC (technically below min specs), 3DMark tells comparable with a RTX 3060 performance wise, but of course not feature wise. Have an older 4k 60Hz display with no special extras.
Whole rig is aging but still doing well.

While playing around with display settings to get a satifying setup I stumbled upon this:
With exactly these settings I found a sweet spot: https://steamuserimages-a.akamaihd.net/ugc/2032867441618765873/91B6A5CEBCFE2162A6340EF0FD6D88CFA3D960C5/

I get a fluent framerate of 30 outdoors and 40-45fps indoors with crisp textures throughout whole game. On 4K with a GPU that does not meet requirements.

BUT:
Whenever I touch one single setting fps drop to 15-20. For example changing shadows or GTAO just one step up or down leads to half performance. Fiddling with upscaling (with this GPU does not even support) ends in 15-20fps.

Can restore this sweet spot reliably this way:
set ultra first, then high and then lowering others. Every other way ends in low fps.

Not omplaining, just looking for some technical insight and explanation of this technical wonder.

Any ideas ?