There was a comparison earlier of the different presets and the resulting performance differences.
Honestly, turn it on 'High', add the top AO solution and you're getting 99% of the visuals of Ultra with a vastly reduced performance cost.
This isn't an optimization problem, this is them including extremely taxing effects at the top settings.
Which is nothing new in PC gaming. I'm consistently baffled how so many PC gamers *still* haven't learned this. I've only been doing this since 2013 but it's all pretty obvious to me.